Timing Contractions

FTM here :) I'm 37w3d and I've been having contractions for weeks now, some painful, some not painful. Tonight I've had a few in my lower back. My question is when do I start timing contractions and do I only count the painful ones or all of them? I'm so use to the non-painful ones that I don't really pay attention to them anymore. Any clue as to which ones I'm supposed to count?