I believe these are some of the simplest methods to figure out the common rafter lengths, how to cut the birds-mouth and plumb cut for any rafter. This method assumes that you will first set your ridge to determine the lengths of the common, hip and valley rafters.You will need a tape measure, string line, framing square, bevel square, saw and of course a pencil.
This method is in lieu of either of the more accepted methods of a Standard 2/3's H.A.P. or the Full Bearing Method. Those methods are explained here. Whatever the width of your rafter stock is, subtract 1-1/2" from it and cut a block of 2x4 that long,. i.e. if yours is 5-1/2" (2x6) cut the block 4". This block will represent the H.A.P. for this roof. If your stock should happen to be less than a 2x6 then only subtract 1".
Place this block on the rafter plate even with the outside edge. Tack it there.
Once you determine the length of the ridge, set it to any desired height you'd like or first determine it's height for a given roof pitch, then tack a string from the ridge to the outside edge of the block. The location of the string on the ridge is not important as long as you make sure that the string is square to both the ridge and plate.
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Now using the bevel square you can determine the plumb cut angle. Place the bevel body against the ridge and align the beam with the string. Measuring from the two points were the string meets the ridge and the block will give you the length of the rafter.
You can follow the same steps for the hip by placing the 4" block at the corner of the rafter plate and then placing the string from the corner of the ridge to the outside of the block and following the steps below to get its length and birds-month cut.
Now crown the rafter stock. Put the crown side away
from you. Now lay your rafter out the following way. Make one mark at the
front of the raft, using the bevel square to scribe the ridge cut. Measure
down the rafter the length you measured and make a mark there. Scribe that
line, this is the plumb cut line, or building line of the birds mouth.
From the top of the bird-mouth plumb cut line measure
down the same 4" you used for the block and make a mark. From that mark
draw a line square to the plumb mark in the direction of the ridge cut.
Now cut the shaded areas and your made a template for
your common rafters. 
Good luck.
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