Before importing, first make sure the .osu file that you receive is in good shape. The most important thing is to make sure that as the person managing the entire beatmap set, for metadata purposes the "beatmap creator" for each difficulty is always
you and not the guest mapper.
When you open the incoming .osu file in Notepad, make sure every line in the [Metadata] section (except Version) matches with an existing .osu in your mapset. "Version" is the diff name which is unique to each diff (duh) and may include the guest mapper's name (e.g. 0108 style, Lasse's Insane).
The .osu file should be named in the following format:
<artist> - <title> (<your name>) [<diff name>].osu
Once the file is good to go, you go to File > Open Song Folder in the editor and just add the file to that folder. You might have to exit/re-enter edit mode for the change to be reflected in the song select screen.
If there are other conflicts between the new diff and existing diffs (like different red timing lines), this would be a good time to resolve them. Use AIMod to check for such inconsistencies.