Don't post the whole pattern please. That would get us into copyright problems. Could you tell us the name of the pattern or link to a photo of the shrug?
Basically, what you're doing is a yo on either side of a center stitch. It sounds like your pattern is asking you to repeat that pattern, that is, work rev st st to 1 st before pm, yo, slip marker, k1, sl marker, yo and repeat rev st st.
(I wonder if it really means "one st before marker" however and not just "to the marker". That's where a look at the photo would help.