Find a good fitting jacket, and one that's un-lined will work the best. so you can see the seams on the inside.
For me the most difficult part of pattern making for jackets is shaping of the sleeve cap and arm hole. I prefer the fit of a two part sleeve so I went with a jacket that had such.
With the paper underneath, stretch and manipulate the jacket to lay flat for the sections you are tracing, taking into consideration the need for seam allowance. I like to make dashes as I go along feeling where the seams are with a pencil.
Then once your done, enjoy the fruits of your labor--sew it up! Keep in mind there might be a lot off about your new pattern so don't use nice fabric, stick to muslim or similar that doesn't stretch so it gives you a good idea of fit.
Pay no mind to the whiskey, I had a good 'ol time getting through this one.
Questions? Bring it.