There are 5 type of flags you can use (and their shortcuts):
- Empty (Ctrl+0)
- Blue (Ctrl+1)
- Green (Ctrl+2)
- Red (Ctrl+3)
- Yellow (Ctrl+4)
You can flag rows 2 ways:
- just click on the sign in the very first column and keep tapping until you see what you want (1)
- right click on any cell of the row you want to flag (2) and select option "Flag" (3) then choose the one you want to use (4)