Chapter 0: Setting up the enviromentBefore you start editing, you should follow these instructions to make our life more easy.
To enable video playback on Reaper you need to download and install VLC media player (x32 or x64 based on your machine and your Reaper installation!).
After the installation you can check if Reaper sees it correctly by clicking on Options -> Preferences -> Video -> Show available decoder information. After checking that you must add the video window to the window layout (View->Window). You can keep it as a floating window or you can dock it somewhere.
Now go on the snap button (the one with a magnet), right click it and a new window should appear. In "Show grid, line spacing" change the box in "Frame". Now the vertical lines in the timeline correspond to a frame, enabling you to cut videos with precision.
Lastly, right click on the video window, go on Performance tweaks->Video processor multiprocessing and put the highest value you can find, and always in Performance tweaks tick the option "Aggressively update video during arrange edit/scroll". This should increase the video playback performance.
You should now find yourself with a setup like the one in the picture below. I like having free vertical space, so I moved the video window and the mixer on the right.