Firstly, if you're doing pe then it's important that you try to standardise your method of measuring. If not, you'll find it difficult to know if you're getting gains when you measure later.
There is still some argument about how to measure length, but most agree that a rule is best and that you should do it whilst standing. With the penis out at right angles (ie. parallel to the floor) place the rule alongside or on top of the penis and gently press the end of the rule into the fat pad that surrounds the base of your penis so that the '0' end is against your pubic bone. See how far the tip of your penis reaches along the rule and make a note of the reading. Metal rules often start with '0' at the extreme end, but plastic or wooden classroom rules usually have '0' a little way in from the end, in which case you can make a correction.
My penis has a slight curve to the left, but I don't really think it's worth trying to allow for that in my measurement. If you have a severe curve, you might prefer to use a tape measure, but you'll probably need someone to help as the tape's end will need to be pressed to your pubic bone above your penis, and the tape will need to be laid (in a curve) along the mid line on top of your penis and a reading taken where it passes the tip of your glans.
Does that make sense?