Also where do I locate your mod in the game folders?

I wrote a guide on how to access mod files here.
Once you got all that going you are free to edit any files (or add files from the base game) and repack it to create your very own mod.

To go by your example of the Ancestral Horn:
The file you seek is records/skills/warfare/ancestralhorn.dbr
Under Spawn Config, you can edit the spawnObjectsTimeToLive entry, which is the time the warriors stay summoned in seconds. Under Skill Conifg, you can edit values like mana cost, cooldown, and level related stuff.
As the warriors itself are summoned creature, they have their own files here:
records/skills/warfare/pets/spectralsoldier(01-20).dbr
Each level has its own entry, so all need to be adjusted separately for values like HP or what kind of skills and resistences they possess. As their standard attack is a skill, you only have to modify that once for all levels to change how much damage they deal. The entry is in the same folder, spectralsoldier_attack_default.dbr.
So to change the amount of damage, navigate to Offensive Parameters > Offensive Absolute > Offensive Physical. That's the amount of flat, physical damage dealt, minimum and maximum. Doubleclick on the string of numbers at the right of either, and a new window will open where you can adjust the values, either by hand or all at the same time. To give you an overwiev of what does what:
Set All: all entries are changed to the same value
Incr. All By: all entries get added the same value
Incr. All By%: all entries get added a percentage of their current value
Incr. By Series: all entries are added a value equal to the entered number x the level (ex: +2/+4/+6/etc)

Save, compile, and test your changes in game. If you have any further questions, feel free to ask.

@ミメミI'm able to create an empty bounce mod, extract the records folder from SkillReworksLP-txt.arz using ARZextractor, copy the records folder to C:\Users\<Username>\Documents\My Games\Titan Quest - Immortal Throne\Working\CustomMaps\<EmptyBounceModName>\database. Then I load up the mod in ArtManager.exe, load the "EmptyBounceMod", select the database tab and go to the warfare subfolder under <EmptyBounceMod>\database\records\skills\warfare. However I am unable to find SpawnConfig. in the ancestralhorn.dbr file. When I right click on ancestralhorn.dbr and select edit, a pop-up says "Error loading record file or associated template".

Hm, you could to try reinstall the templates in the Art Manager: Help > Install Templates...

Also why is it that the font of the name of the map in faded yellow when one is in the custom game mode?
Managed to save and tested out the changes.

How does one make the skill "Summon Outsider" permanent? Like other pets such as Lich, Wolves, etc

For the "Blood of Ouranus" skill granted by the weapon Blood of Ouranus, how do I find the dbr file and edit the skill?

I'm also unable to edit the HP of the ancestral warriors. I tried editing the entry characterLife spectralsoldier_16(to20).dbr in the subheader CharacterBio\CharacterParameters\Character Bonus Attributes, but the edited HP is not reflected in game after I have saved the changes and build it in ArtManager.

Saving my BKB for Dota 3 の投稿を引用：

Also why is it that the font of the name of the map in faded yellow when one is in the custom game mode?

No idea, it's pale blue for me.

Saving my BKB for Dota 3 の投稿を引用：

How does one make the skill "Summon Outsider" permanent? Like other pets such as Lich, Wolves, etc

The easiest way is to make it pseudo-permanent by raising spawnObjectsTimeToLive (in SpawnConfig) to 30000 or something, this should be enough for a long game session.

Saving my BKB for Dota 3 の投稿を引用：

For the "Blood of Ouranus" skill granted by the weapon Blood of Ouranus, how do I find the dbr file and edit the skill?

You can check the item's file for a referral to the skill in question (under Skill Augment). For the Blood of Ouranos its records\skills\item skills\bloodnymphsummons.dbr, which in turn refers to records\skills\item skills\pet\darknymph_10.dbr.

Saving my BKB for Dota 3 の投稿を引用：

I'm also unable to edit the HP of the ancestral warriors. I tried editing the entry characterLife spectralsoldier_16(to20).dbr in the subheader CharacterBio\CharacterParameters\Character Bonus Attributes, but the edited HP is not reflected in game after I have saved the changes and build it in ArtManager.

Is that the name of your dbr? If yes, the "(to20)" has to go, the file always has to have exactly the same name as the vanilla one. Apart from that, did your character actually had skill level 16, or did you have any +skill items?

I was referring to spectralsoldier_16, spectralsoldier_16, ....., spectralsoldier_20. Yes I have +skills items that raises the skill level by 4.

As for making summons permanent, I figured out that you just have to delete the values in spawnObjectsTimeToLive to replicate to that of other summons such as the Liche King.

How does one make Phalanx units mobile e.g. similar to Ancestral Warriors?

Saving my BKB for Dota 3 の投稿を引用：

I was referring to spectralsoldier_16, spectralsoldier_16, ....., spectralsoldier_20. Yes I have +skills items that raises the skill level by 4.

Huh, should work in that case. Did you make any other changes so far that did show up ingame?

Saving my BKB for Dota 3 の投稿を引用：

As for making summons permanent, I figured out that you just have to delete the values in spawnObjectsTimeToLive to replicate to that of other summons such as the Liche King.

Good to know. I vague remember this didn't work or caused problems in the past, but either that's not the case anymore or I remember wrong.

Saving my BKB for Dota 3 の投稿を引用：

How does one make Phalanx units mobile e.g. similar to Ancestral Warriors?

Uhh...while I believe that it's entirely possible to make it work, it would probably be easier to change that skill to summon ancestral warriors that look (and attack) like phalanx soldiers. That's some serious work, though.

ミメミ の投稿を引用：

Saving my BKB for Dota 3 の投稿を引用：

I was referring to spectralsoldier_16, spectralsoldier_16, ....., spectralsoldier_20. Yes I have +skills items that raises the skill level by 4.

Huh, should work in that case. Did you make any other changes so far that did show up ingame?

Saving my BKB for Dota 3 の投稿を引用：

As for making summons permanent, I figured out that you just have to delete the values in spawnObjectsTimeToLive to replicate to that of other summons such as the Liche King.

Good to know. I vague remember this didn't work or caused problems in the past, but either that's not the case anymore or I remember wrong.

Saving my BKB for Dota 3 の投稿を引用：

How does one make Phalanx units mobile e.g. similar to Ancestral Warriors?

Uhh...while I believe that it's entirely possible to make it work, it would probably be easier to change that skill to summon ancestral warriors that look (and attack) like phalanx soldiers. That's some serious work, though.

Yes for example I am able to tweak the number of Ancestral Warriors spawned, but as for the hp of the warriors the skill description is fixed at 2400hp.

Deleting the values in spawnObjectsTimeToLive trick does not seem to work on skills such as Rally though.

Yes I expected overhauling Phalanx units to exhibit Ancestral Warriors behaviours will be a pain in the a$$.

How do you re-introduce the "shotgun shrapnel" multiple meteors back into the meteor skill from the Earth mastery? I've only managed to add in fragments and tweak the drop offset.