this is the easy part, if you have made any custom skins, you (hopefully) made a backup of your old pak01_dir.vpk file, if you did, then rename it to pak01_dir.vpk.old

and if your current custom file is named pak01_dir.vpk, copy it and name the new one: pak01_dir.vpk.hax

both should be in your csgo folder.
for me it's "C:\Program Files (x86)\Steam\SteamApps\common\Counter-Strike Global Offensive\csgo"

open up notepad, and pase in the following block of text
----------------------------(without this line)--------------------
@echo off
echo Switching to custom weapons mode...
cd "C:\Program Files (x86)\Steam\SteamApps\common\Counter-Strike Global Offensive\csgo"
del pak01_dir.vpk
copy pak01_dir.vpk.old pak01_dir.vpk
echo done.
pause
------------------------(and without this line)----------
if your cs go folder is somwhere else, edit line two to your correct path.

now save the file as: normal_go.cmd
(you name it watever you want, as long as it ends in .cmd or .bat)

running this file will switch you to the default seup.

now open an new notepad window and enter this:
----------------------------(without this line)--------------------
@echo off
echo Switching to custom weapons mode...
cd "C:\Program Files (x86)\Steam\SteamApps\common\Counter-Strike Global Offensive\csgo"
del pak01_dir.vpk
copy pak01_dir.vpk.hax pak01_dir.vpk
echo done.
pause
------------------------(and without this line)----------
again, if your cs go folder is somwhere else, edit line two to your correct path.

now save the file as: custom_go.cmd
(you name it watever you want, as long as it ends in .cmd or .bat)

To run the switcher, simply close notepad, and find where you stored the files.
then double click the file that you want to use 'custom_go' for custom weapons or 'normal_go' for normal weapons. then launch the game.

note: if you want to have the game launch as soon as you run the switcher, read the next section
From now on, if you want to add any custom weapons, I would suggest editing the pak01_dir.vpk.hax file, then running custom_go. this way you only have to edit one file, and if a CSGO update changes pak01_dir.vpk, you'll still have a backup, you can easlly switch to!

Here are some optinal thing you can do to make the switcher better!

Make the game launch as soon as you run the switcher.
right click on the one of the switchers, (dose not matter which one) then click 'edit'
and after the word 'pause' press enter
and enter the following:
"C:\Program Files (x86)\Steam\Steam.exe" -applaunch 730

If you want to be extra safe, add this insted to your custom_go file:
"C:\Program Files (x86)\Steam\SteamApps\common\Counter-Strike Global Offensive\csgo.exe"
that will lauch the game outside steam, and you will bypass file checks (but you will not
be able to join any vac secured servers)

(with the quote marks included) then save the file, and do the same with the other one.

now when you run the swticher, it will switch your weapons, ask you to press enter (so you can see if you got any errors) then launch the game!

Stop asking me to 'press any key'!
if you want the switchers to stop prompting you,
just edit both files, and remove the line containing the word: pause and save them

Black boxes scare me! how do I change the color of the switcher's window?
edit both files, and after 'echo off' press enter,
then add the line: color fo
(do this to both files)
then save them, now when ever you run the swithcer, it will show a box that looks more like notepad.