Shaping saddle on onesie

Pattern:https://www.etsy.com/listing/205192949/no324-baby-romper-pattern-pdf-vintage

“Shape saddle: work even in pattern until piece measures same length as shoulder, stretching slightly.”

From what ive read, if im shaping a saddle that means that the “saddle” is going to go between where the front and back shoulders are normally sewn together? If not, what is a saddle?

Where am i measuringon both pieces? On the sleeve, am i measuring the entire project? Even though rows before you start decreasing on a sleeve dont end up getting sewn into the armhole so??
And then on the front or back im measuring the armhole and up over the shoulder decreases, stopping at the neckline?

Sorry if that sounds confusing but thanks so much if youre able to help me here!!

You’re right. The saddle is going to go between the front and back shoulders. It’ll be seamed to the front on one side and the back on the other side.
Measure the length of the saddle by measuring along the top of the shoulder (where the front and back would ordinarily be seamed together).
Really cute pattern.