This tutorial is by Mrvicho13 But it was re-written by me due to him just deleting it from GTA forums
First things you'll need
General knowledge of how to use 3ds max
3ds max. (you can get a free student download) from their website.
GIMS IV for 3DS max Credits to 3Doomer
Install GIMS IV to 3ds Max using the instructions from the .rar
OpenIV Credits to OpenIV Team
Kams GTA scripts for 3ds max Credits to Kam
Install Kams Max Script using the Must_Read.txt from the .rar
Script hook V Credits to Alexander Blade
Fivem Credits to NTAuthority
Fivem converter Ui Credits to Link/2012
ytdydryddyft2txddffcol Credits to DK22Pac
And of course patience (possibly lots of em') .
Install the 3ds scripts to the locations it tells you to in the .rar
Create a Folder at your desktop with any name and extract the ytdydryddyft2txddffcol tool, FiveM and the Fivem UI. (Like this)
Open 3ds max and IMPORT the model that you would like to add to GTA V. (in this case i'll be using the Shock hammer from Saints Row The Third ). Once imported your 3DS max your viewports should look like this
Start OpenIV and go to (update/x64/dlcpacks/patchday3ng/dlc.rpf/x64/models/cdimages/weapons.rpf)
Extract the the weapon you want to the ytdydryddyft2txddffcol\converter folder. (Skip this step if you're replacing a melee weapon)
Drag and drop the weapon you want on to the ytdydryddyft2txddffcol.exe. Once dropped you should have a CMD screen appear. (Skip this step if you're using a melee weapon)
Once converted to a .dff import the model using Kams gta script (If using a melee weapon download and import this reference model credit to Mrvicho13) Also make sure the A is checked of like shown in the picture.
When you have imported the model scale your custom model down to match the dff (DON'T MESS WITH THE DFF's SCALE ONCE IMPORTED!) Obvously the more accurate the size and shape the better it'll look ingame.
Once you have scaled the model down select the reference select "vertex" from the box to the right of the screen then press Ctrl+A. The blue dots should now be red. Now what you'll need to do is right click the custom model and attach it. Once you have attached it just press Delete and all that should be left is your custom model.
Now it is time to edit the materials. Now you may start GimsIV from the hammer icon from the right side of your screen.
Select your model and press the "Convert materials" then go to "Material editor" open the material editor and select the folowing.
Now a new window should open like this
Now close the Material editor and open the Hierarchy manager and type in the name of your model in the highlighted area
Go to the top of the GimsIV tab and click OpenFormats. Next to Export ord/odd there will be an asterisk click it and deselect split smooth groups.
Start OpenIV and select" GTA IV" when it asks you to locate the folder, Select the location that you placed your GTAIV.exe in. Once you have selected the location of your .exe Select "Edit mode" and then Shift+ins. and this window should now pop up.
Once you have done that open CMD and type in the following Be sure to type in YOUR LOCATIONS AND MODEL NAMES!