-Find the .vpp file for the map you want to edit (for custom maps, this will almost always be in RedFaction\user_maps\multi, and for stock maps, it will be either the "levelsm.vpp" file or "levelspf.vpp" file in the root RedFaction directory)
-Open that .vpp file in VPPBUILDER, locate the .rfl file for the map, hit the "Extract" button, and extract it to your RedFaction\user_maps\multi folder. If the map relies on any custom textures (.tga or .vbm files inside the .vpp file), you should extract those as well. Custom textures go in your RedFaction\user_maps\textures folder.
-Open REDPF (if you had it opened already, you will need to restart it), and use the File->Open feature to browse for and open the .rfl file you just extracted.
-Use the File->Save As feature to save the map immediately under a new .rfl name. If you don't do this, it
will cause compatibility issues when you try to do anything with your map.
I should note that as a matter of common courtesy, if you want to edit another person's map, you should ideally obtain their permission before doing so. In a game such as RF, where 99.99% of mappers are no longer involved in the community (and pretty much impossible to contact), you can probably feel comfortable just changing the "Author" attribution field in Level Properties to something like "Goober (Edited by OvalPistol)", and clearly stating that the map is an edit if/when you go to upload it to Faction Files for others to play.
As far as general RED/REDPF help, we've got a selection of tutorials in the tutorials section of our forum:
[Link]. Other than that, you can check out the official RED help files from Volition:
[Link]
If you have any additional questions, feel free to post on our forums about it and myself or another mapper should be able to provide an answer.
Welcome to Faction Files, and happy mapping!