1) Write down all materials. Ensure materials match and make sense. For example... The Yachting Jersey calls for "... the best unshrinkable Alloa or 5-ply fingering." First, Alloa yarn was woolen and decidedly NOT unshrinkable*. Next, a 5-ply yarn is Sport Weight yarn according to most yarn weight standards, not a Fingering yarn. Hmmm. Write it down and move on.
Elements which do not make sense must be looked at in the context of all the other elements of the pattern. If, for example, the pattern has contradictions about yarn weight (as above), you may use the needle size as a clue as to which is correct. Or sizing... if the yarn weight , say fingering, will result in a 12" wide sweater designed for a man, it may be wise to bet on a conflicting element of the pattern (say, needle size) which would tend to produce something larger. Write down ALL the MATERIALS, and then look for what makes the most sense... interpreting historic patterns is an art, not a science!
2) When all else fails, search the pattern for something which indicates finished object measurements. When these data bits appear, hold on to them for dear life. If you are aiming for a 13" cast on length, and your cast on with size 1 needles and fingering weight yarn totals 6"...ADAPT. Knitters did not always have hundreds of needles from which to choose and made do with the needles in their houses. They adapted their materials to make what they needed. The modern knitter, when faced with nonsensical patterns must be willing to do the same. Written instructions were meant as a guide, not law.
3) "Plain knitting" in historic terminology means GARTER stitch (not stockinette, and not purl). It is sometimes abbreviated P1 for "plain knit." I'll admit, this had me pulling out my hair as I read and reread the Yachting Jersey pattern. Thank you, Franklin.
In summary, when knitting from an antique or vintage pattern, FLEXIBILITY IS AS IMPORTANT AS PRECISION.
Thank you, Franklin, for helping me move past my considerable OCD barriers...and for a fun morning workshop!
* Viyella, a blend of merino and cotton, was available in yarn form, but it is unclear to me whether it was being made prior to 1920.