Installing Mods with OpenIV
Install OpenIV. After you installed OpenIV, start it. Click on GTA V, then choose your GTA V folder. Now go to "Tools", then click on "ASI Manager" and install everything there. Now you can install every .oiv file with just a few clicks. To install a .oiv file, go to "Tools", then click on "Package Installer" and choose the .oiv file. I would recommend using the Mods folder.

Watch this video if you couldn't install OpenIV

Installing SkripthookV & SkripthookVDotNet
You have to install SkripthookV & SkripthookVDotNet.

After you installed SkripthookV and SkripthookVDotNet, you must create a folder in your root GTA V folder and call it "scripts". Drag the scripts into the "scripts" folder. And then you are done!

Watch this video if you couldn't install SkripthookV