Loot mod is a mod for Serious Sam Fusion that makes enemies drop weapon/ammo/health/armor and powerup items on death, and loot can be configured to your preference.

First, you need to install Loot mod and required mods (Weapon Resources Database and Network Script Resources). Then launch the game, go to "Settings->Keyboard and Mouse->Configure keys" in game menu and assign "Open/Close CWM/RWS/Loot menu" command to some key (if you already used CWM or RWS menu before, this is the same command).

Don't assign it to something that you can press during gameplay or chat, like a letter button. Assign it to something safe, like any of FXX buttons. For example, I use F4 button for menu.

Now you're good to go, start any level and begin configuration!

Press the key you assigned to menu button on any level. A menu should show up (like on screenshots on mod page, but it will be CWM/RWS menu by default if you have those installed). If you have CWM/RWS installed, disable them before using the Loot mod, otherwise it just won't start (CWM is disabled by clearing all weapons).

Loot mod is disabled my default, you should enable it in corresponding menu, and restart level for it to take effect. Enabling/Disabling is the only Loot mod action that requires level restart, all other changes will take effect right away.

So now you have several things to configure. First I'd recommend to open "Set probabilities" submenu. Here you can configure 4 parameters: "Ammo&Health probability", "Average rocketeer drop", "Average Biomech major drop" and "Maximum drop probability for each item".

Let's start with average drops. Every enemy has an "Average number of dropped items" parameter, which is calculated based on monster's total health.

These two (Average Rocketeer drop and Average Red Bio drop) parameters define how many items (on average) will rocketeer and red biomech drop on death. Then, based off these two numbers, Loot mod forms linear dependency, which is used to calculate average drop number for any monster depending on its health. For example, if "Average Rocketeer drop" will be set to 0.19, and "Average Red Bio drop" will be set to 7.5, then "Average drop" parameter for Kleer will be 1.25.

Now onto "Maximum drop probability for each item".
Consider Biomechanoid major, and assume its "Average drop" parameter is at 7.5.
Then, if "Maximum drop probability for each item" is set to 100%, then on death Red Biomech will drop 7 items wth probability 100% each (so 7 guaranteed items) and one more item with probability 0.5 (50%). If you change "Maximum drop probability for each item" to, say, 75% (0.75), then there will be 10 (=7.5/0.75) possible items dropping from Red Biomech, but each of these 10 items will be dropped with probability 75% (and not dropped with probability 25%). So you would have a chance to get up to 10 items, but also a chance to get less than 7 items (even 0).
Basically this parameter changes dispersion of the number of drops for bigger monsters. If you make it 100% or close to it, amount of dropped items will be consistent, if you lower it, you will get more chaotic drops.

Lastly, "Ammo&Health probability". This is pretty self-explanatory. When an item is dropped, it can be either a health/armor item, or a weapon/ammo item. This parameter balances amount of health/armor and weapons/ammo you get. If it is set to 0%, you won't get any health/armor. If it is set to 100%, you won't get any weapons/ammo, only health and armor.

Now let's get to configuring our weapon loot table. You can add weapons from "Add weapon" menu, edit already added weapon weight from "Config" menu or simply load some existing configuration.
What's this "weight", how it works?
Consider a simple example: your loot table has a Cannon with weight 1 and a Devastator with weight 2. Total weight of all weapons in the table will be 3. This means, that whenever weapon/ammo item is be about to drop, with probability 1/3 (33.(3)%) it will be Cannon-related, and with probability 2/3 (66.(6)%) it will be Devastator-related.
Now assume you added an Assault Rifle with weight 7 to this table. Now total weight of all items is 10 (1 - Cannon, 2 - Devastator, 7 - Rifle). So Cannon will drop with probability 10%, Devastator - 20% and AR will drop with probability 70%.
I hope this is understandable. If not, "Probability" parameter is always displayed when you configure weight.
And you can always just add all weapons to the table with same weight, this will make them all equiprobable.

Another two options in main menu is "First player picks/All players pick" and "Item Block".
"First player picks/All players pick" switch defines whether dropped items will be pickable by all players in co-op or only by the first one.
"Item block" has 4 states, "Off", "Ammo/Weapons", "Health/Armor", "All". When "blocked" option is selected, all items of chosen type that weren't dropped from monsters become unpickable, so that you can fully immerse yourself in loot-only item obtaining and safely ignore all items that were already on the level. To pick a "blocked" item, you can either disable blocking in menu, or simply stay in the center of a blocked item for 2 seconds, and it will become pickable.

Lastly, there is a "Remove Weapon" submenu. It does exactly what it says - left clicking on weapon in it removes weapon from your inventory, clicking with right mouse button removes weapon and all ammo for it from your inventory. This is added in order to remove unwanted items that game gives you.
For example, you decided to play in HD with BFE weapons, you configured loot table with BFE weapons, pick and use only them, then you end the level and in the beginning of the next level game gives you a bunch of HD weapons without asking. You may want to get rid of them.
This button works in Multiplayer too, and even if Loot mod is disabled, but requires for the host of the game to have Loot mod installed.

If you want to use console to configure the mod (or the menu just isn't working for you for some reason), there are console functions to do that. Usage is mostly the same as in CWM mod, so you can refer to CWM guide if you got problems using these.

globals.EnableLM(X) - enable (1) or disable (0) Loot mod;

globals.LMFirstOnly(X) - set whether only first player (1) or everyone (0) will be able to pick dropped items in MP;

globals.LMItemBlock(X) - set whether non-dropprd items will be blocked: 0 (no block), 1 (weapons/ammo blocked), 2 (weapons/ammo/health/armor blocked) or 3 (health/armor blocked);

globals.LMSetMaxProb(X) - set maximum drop probability for each item (see previous guide page for details);

globals.LMSetProbabilities(X,Y) - set drop probabilities so that Rocketeer drops X items on average and Biomech Major drops Y items on average; basically X and Y are "average rocketeer/red bio drops" from menu;

globals.LMSetAHProb(X) - set probability of health/armor drop;

globals.LMList() - list of all available (for you) weapons.

globals.LMAddWeapon(X,W) - add weapon X to loot table with weight W;

globals.LMRemoveWeapon(X) - remove weapon X from loot table;

globals.LMConfig() - show loot table and all configured parameters;

globals.ClearLootTable() - clear loot table;

globals.LMSaveConfig('NAME') - save current loot table under name NAME;

globals.LMLoadConfig('NAME') - load loot table with name NAME;

globals.LMDeleteConfig('NAME') - delete loot table with name NAME;

globals.LMConfigs() - see all configs you have.

Here is a collection of random facts about Loot mod that don't deserve own section in the guide.

• As mentioned before, all parameters (except turning Loot mod on or off) can be changed on the level and changes will take effect immediately, without level restart;
  
• Randomization goes as follows: for every item that should drop, first it is decided if it will be health/armor item or not. If it is, then some more internal rolls define type and size of health/ammo. If it is not a health/armor item, there will be a small chance that it will be a powerup. If it's not a powerup, then the loot table is used to roll the weapon type;
  
• For every weapon in the loot table, the weapon will drop at least once before any ammo for it can drop. After weapon item has been dropped at least once, almost all drops for that weapon will be ammo, and only with small probability (which is increased if some player on server doesn't have the weapon in inventory) this weapon's item will drop again.

Have fun and report bugs in the comments!