If you have a clean installation of FTL you can just install slipstream mod manager, the mods and activate them.
If you're not sure that your FTL installation is clean keep reading.
"\Steam\steamapps\common\FTL Faster Than Light\Slipstream Mod Manager v1.4-Win\backup"
and delete everything in the backup folder, if u activated any mods after the update, make sure your have the latest "data.dat" and "resource.dat"
if you're not sure, delete those 2 files in:
"\Steam\steamapps\common\FTL Faster Than Light\resources"
then verify game files (this will download the latest "data.dat" and "resource.dat" files) , start the mod manager and activate BetterPlanetsandBackgrounds 1.2.1 or 1.3.1 and optional better asteroids....