Fir length: Stand up, and hold your erect penis straight out from your body (horizontal). Place the '0' of your ruler at the base of the top of your cock. Press the ruler gently into the fat pad so that it rests against your pubic bone and see what number the tip of your cock reaches. That is your bone pressed erect length (BPEL), which is the method most people use.
For girth: Wrap a dressmaker's cloth tape around your erect penis and see what the girth measures. You may be thicker at the base, or round the mid shaft, etc. depending on the shape of your cock, so measure it in several places. Some surveys want you to average the measurements and come up with a 'mean' girth measurement, but others want you to state your maximum girth (whichever part of your cock that is). Because of that I take 'average' girth stats with a bigger 'pinch of salt' than length measurements.