how do I play a piece of classical guitar when the fingers (pima) are not indicated, do I just figure it out??
Playing guitar does not have to involve strict adherence to someone else's ideas about how a piece should be played. The guitar as an instrument, lends itself quite well to creative interpretation and adaptation.
When someone writes the notation for a piece of guitar music and adds the finger indications (P I M A), those can be treated as "suggestions". The suggestions often have some merit and might represent the easiest and most efficient way for most guitarist to play the piece. But you will be playing the same notes regardless of which finger you use. So if you find it easier to play a particular note with your index finger than your ring finger, you can play it that way - regardless of the "suggested" finger.
So to answer your question - yes. In the absence of someone notating their suggested fingering, you simply figure out what works for you. You may see that the notes can be played using a finger picking pattern that you are comfortable playing. If so, use that pattern. Typically the bass strings (5th and 6th) will be played with the thumb and the middle and treble strings with the index, middle and ring finger, depending on the pattern.
Some guitarist rarely use their ring finger, preferring to use only the thumb, index, and middle finger and seem to play quite well. So they would generally ignore any "suggested" fingering that included the ring finger.
You might need to experiment - measure by measure, to see what you are most comfortable with. Then you can write the P I M A notation yourself if you feel the need to memorialize what you work out.
Good luck and keep it fun. And don't be afraid to experiment.