I can't describe how to do the long/short/regular bit, but I can give you the way to measure for the "number part"

It's MUCH better to have someone else measure you... You can do the following WITH a t shirt on.

Put your hands up and have someone wrap a tape measure around your chest right over your nipples (generally the widest part) and around the back. Put your arms down with the tape measure still around your chest. Take that measurement and add 2 inches.

Keep your arms down and have someone wrap the tape measure around your arms AND chest at the widest point. Take that number and subtract 6 inches.

The 2 numbers you have SHOULD be within 1 or 2 inches of each other. Take the bigger number and that's the size jacket you take.

i.e... I measure a (40 +2 =) 42 around my chest and (50 -6 =) 44 around my chest AND arms. I would take a 44 Regular.

This is how measurements are taken at "After Hours" tuxedo rental here and that's how we take them in the shop. It seems to work very well!

Hope this helps!