It’s not that difficult if you’ve been sewing for any length of time, and they fit much better than an altered commercial pattern. You need a few tools, like a couple of rulers (one short and one longer, like a yardstick) a French curve, a hip curve, pencils, measuring tape, and patience. Oh yes, you’ll want cheap paper for starting out and then when you get the fit perfected you might want to redraw the pattern onto Swedish tracing paper (available online).
There are two books I recommend; 1) The Pattern Making Primer by Jo Barnfield and Andrew Richards, and 2). Fast Fit by Sandra Betzina. The second one I got used on Amazon for cheap.
Fast Fit has been really helpful for me lately, because my daughter has “grown up”, and is much curvier than I ever dreamed of being. I’ve had to redraft her patterns three times in the past 3 years (whew!!). Last week I took her regular dress pattern and re-drafted it into a princess seamed dress. It looks terrific on her and uses slightly less fabric than just a regular sheath or shirt-dress.
One more thing–there is LOTS of help on YouTube–just search “drafting long sleeves” or whatever part you’re working on. I love You Tube. You can learn just about anything on there!