Just measure the pattern
First you need to measure yourself. I would say you need at least:
- shoulder lenght (from neck to end of clavicle; if you cannot feel the end of your clavicle, you can measure to the point were your underarm line would meet your shoulder)
- bust (+ underbust if your measurements differ significantly)
- waist
- hips
- front lenght (from point in between clavicles to waist)
- back lenght ( from poking bone on bottom neck - sorry, dunno what it's called
but it sort of pokes out on everyone to waist)
You then measure on your pattern or use the finished measurements if they are written on the pattern.
Bust guide is usually a line squared from center front to end of armhole. The measurement on the pattern should be your measurement + ease (normal ease is between 2.5 and 5 cm, some people like more, some like less). What is over this is design ease (you can check the drawings on the envelope for how loose the garment is supposed to fit and if you would like to add/substract ease). Same for waist and hips (hips usually have a bit more ease to allow you to sit down).
Front lenght/back lenght and shoulder seams are a little bit more tricky to measure because of design.
You could measure on yourself and pattern from shoulder point at neck to bust line and from bustline to waist, but I find this is not always accurate.
Most times, when you draft a pattern using your own measurements, you still have to do a lot of fitting, because we are moving people not dress stands and have different issues and volume distributions.
You could also try drafting and fitting your own sloper and using that to alter commercial paterns.