Do it as soon as u start feeling the contraction right at the beginning an then time when u feel the onset of the next one. If you do it when its paunful ur going to.get an inaccurate time becaue one contraction.can start more painful.then the next and ur timing will b all off and u may think its irregular when it actually could.be regular. Also make sure they are increasing in pain as thats a good indicator as well.
I have an app I've been using.. Says to start at one and then stop when it's done. Calculates the times in between.. But I've always started it when the contractions start getting strong
they also have an app you can get on your phone for free and you just push start when you feel one and stop when it stops and it tells you if it seems real
You do it from the start of one contraction to how long it last and then when the next one starts
Do it from the start of one contraction to the start of the next one. That way u get the most accurate time.