If you're having compatibility issues while using RDE and LSPDFR, do the following:
Create a clean game folder. One without any plugins or modifications. That also means no Rage Plugin Hook. If you don't have a backup, you'll have to download and install the game again, or manually remove all your modifications. Just make sure you actually have removed everything.
Now make sure the game is up to date and launches without any issues. If you're using steam, launch it through steam.
Install the ASI loader and OpenIV.asi through the OpenIV ASI Manager. Then install ScripthookV, ScripthookVDotNet, and the RDE oiv file. When building the oiv file, I've found that selecting the custom files for LSPDFR-related modifications does not work, and because of that I'll tell you how to add them manually further down.
Please note that selecting files for CopHolster or Simple Native Trainer does work, but that you'll have to install these mods later without overwriting these files. Keep that in mind.
Now make sure the game launches without any issues, and that RDE works. Again, if you're using steam, launch it through steam.
From there, go ahead and add LSPDFR and Rage Plugin Hook (in that order!). Also, go into the scripts folder, and remove everything. These scripts has to do with Six Star Response, Arrest Warrant etc. and does not matter if you're playing LSPDFR. Neither are they compatible.
LSPDFR should launch without any issues. This time, of course, launch the game using Rage Plugin Hook. If you've made LSPDFR auto-load on launch, and it crashes on launch, please try to load it manually through the console. Hit F4 (which is the default) and type in LoadPlugin. LSPDFR should pop up as the one and only choice. My game has no problems running RDE and LSPDFR together this way, and neither should yours.
Now onto the LSPDFR-related files that couldn't be added automatically..
Install the modifications you want. Do it one by one, though, and launch the game after each time to make sure it all works. It takes time, but that's modding.
Find your RDE oiv file, the one you installed earlier.
Right click, click "Open" on the very top, and select WinRAR. If you're using 7zip instead of WinRAR, I would assume it is also compatible, however I've never used 7zip, and I truly have no idea. If it doesn't work, get WinRAR.
Inside the oiv, open the "content" folder.
Here there's 6 folders starting with "LSPDFR", followed by the specific mod the folder contains files for. You'll have to figure out where these files go yourself, however it's gonna be inside your Plugins folder somewhere. Look through the folder for the modification you're installing files for, and you'll eventually find them.
Please note that there are folders named "FBI" inside some of the 6 "LSPDFR" folders in your oiv. These are supposed to be used instead of the ones outside of the FBI folder if you've installed the Federal Law addon.
I hope you get it working, and that you can enjoy RDE agencies, departments, vehicles and peds with LSPDFR! If you're having issues, I recommend joining the RDE discord server and message us in the suggestions-support channel! Link is further up and on the RDE download page.