Press firmly first time and you will not have to go back anyway. You should listen to the sound the cutter makes as it travels across the glass to hear if you have missed a small part of the cut. If you do miss a bit, FIND IT and just mark the bit you missed. DO NOT go back over the parts you have already marked. If you miss just one tiny part and the score line is not continuous, the glass will not follow the line and the glass will not break the way you want it to.
Most beginners get to the edge, hesitate, and then stop, just before the edge of the glass.
SURE your glass cutter firmly drops off the edge and onto the cover of your glass-cutting table. Now you know the tricks of the trade, go and practice, practice, practice.