MisterPfister

The best way is to first open GTAV, enter Story-mode, when you have a stable story-mode with free roaming, then use Alt-TAB, goto desktop and there start RPHook.
Then use Alt-TAB to jump back to GTAV and wait for LSPDfr to open completely.

A more detailed way to do this:

Special situation
1. run after an R* update
After you have entered story-mode as normal RP-Hook will react a bit different.
First you get a warning from windows defender
It says that win-defender has protected you from a *potential* dangerous executeable
click More Info
Scroll to the right
Choose option run anyway

In GTA nothing seams to happen
Go to tray
RPHook has now a disclaimer-popup
Click the OK to accept the EULA
We are almost there but RP-Hook will now open a pop-up in tray Dispaying all the files that has been blocked by windows

Here you choose Yes to unblock all files
Now RP-Hook opens the std UI. Here you should check that plugin-timeout is 60000 ms, and then you should chose the option Save-and-Launch.
Now You will see a message that RAGE is hooking -as it should and shortly after you will see that LSPDFR is running inside GTAV
From here after ...until the next R* update- The startup sequence is again normal and as above.
 

*facepalm* of course that's what I forgot. Thanks a bunch!

Install RageNativeUI, download from here: https://github.com/alexguirre/RAGENativeUI/releases
drag and drop the RageNativeUI.dll in the main directory