Installing LSPDFR is as simple as drag and drop. You then need to go to https://ragepluginhook.net/Downloads.aspx to get the latest version of ragepluginhook which is also drag and drop and replace files. Most callout plugins are also drag and drop into gta 5 main directory, if a mod requires any different it will tell you how to install.
Vehicles are not hard to install as it mostly involves replacing models and changing lines in certain "text files". There are some vehicles that do not have a certain model they replace and is up to you to decide. This type for me is the "hardest" to install as you have to change the "text files" provided to the certain model you choose. There are even vehicle mods that come in the form of an automatic installer, my favorite pack to use happens to be one of these and can be found here
So most other mods are drag and drop/replace and nearly all mods have good installation instructions.