Requirements: All of the software can be found here: http://ravenfieldgame.com/modding.html#links. You will also need Ravenfield.

Ravenfield modding is a pretty easy thing once you get the hang of it, making quality things is the hard part. I'm not here to show you how to make quality, I'm here to show you how to make anything at all. There's a million other, better guides out there, but I'm making a fictional pistol for this guide's purposes. The first thing you're going to want is a model. This step isn't necessary, as my earlier mods have shown, but unless you want to be making weapons which are mish-mashed parts of other models, like so: Then I recommend making a model. Asset flipping (Just taking a model that you didn't make and using it) is also possible, but no one likes asset flippers. Save this model in your RF tools assets folder, I usually save mine in Assets>Models.

Your next step is to go to the Weapons Lab in RF tools. If it doesnt work, I can't help you. I literally uninstalled and reinstalled repeatedly until it worked.
This is what it will look like upon loading up. Don't be intimidated, this is much easier to work with than it looks.
For the purpose of this guide, we are making a pistol. So I unchecked the AK, and checked the pistol. My pistol is green for some reason, but yours won't be.

As you can see, there are 2 MK25s here. The lower one is the actual model and such, the top one houses the weapon configuration.
Step 1
Drag your model out of the asset folder you saved it in, into the 2nd MK 25. Scale will usually be weird, if you don't know how that works, like me. Adjust the scale, rotation and transform. For this guide's purposes, I set it up already.
There are parts of the model which work with the animation controller, these being the Slide, Hammer, and Magazine. The muzzle is for muzzle particles and for where the projectile is spawned, and casing particles are self explanatory. If these are conflicting with how you want your model to look, either click the cogs over their mesh renderer or mesh filter component, and remove the component, uncheck them, or remove the mesh from the mesh filter. Try not to delete them.
If you want to move part of your model, do not move parts of the default weapon. They will snap back to their original location if you do so, and complicate things.

Note how my pistol lacks a visible hammer. I cleared the mesh filter. Clearing the mesh renderer results in pink textures.
Note that if you are using another weapon as a base, it will likely have different animated parts.

Here comes the last part, configuration and export. For the configuration, mess with the values to see what does what, and what fits your weapon the best. There's probably another handy guide which outlines it all, but your basics are going to be the fire rate, spread, ammo, recoil, and tracer. Weapon sounds are also important, but can be repurposed easily with pitching. Fire rate and ammo are self explanatory, spread dictates how much your projectile is going to deviate from where you're aiming, recoil is divided into kickback, and snap. Snap is how far it "snaps" vertically and for how long, kickback is how much the gun kicks backwards. The tracer is the important part, As you'll likely need to copy a tracer from Assets>Prefabs>Projectiles, and edit it to your heart's content. There are 3 rifle tracers, a few rockets, a pistol tracer, and more, but I edited the base pistol tracer to act as the 6.57mm round used by this pistol. Once your weapon is done, all you have to do is drag it down into your Assets folder, somewhere, preferably in Assets>Prefabs, or a folder of your making still in the assets folder. Once you've dragged it down, it is saved. Personal preference, I never save the Weapons Lab scene, but if you do save over it, the vanilla weapons are in the Prefabs folder. Once that is done, you need to export it to Ravenfield. Set game executable (Where Ravenfield is on your computer, not a shortcut), then you need a content mod.
Copy one of the example content mods, fill out the fields, and you're golden. Make sure to have your weapon prefab saved in the Assets folder, and have it in the prefab slot of the content mod. Then, simply save the content mod somewhere in your assets folder, and Ctrl+E to export it when it's selected.

This was a crude guide on how to make weapon mods for Ravenfield. More advanced guides are recommended after this.