I don't know of many patterns that do write out both sides. Basically it means do the BOs and decs at the other edge of the front.
For the L front you probably bound off a few sts at the beg of a RS row for the underarm, then decreased right above them. For the R front you can't BO at the end of a row, so you need to BO on the WS row to put the underarm at the other edge. Then you can dec at the end of the RS rows instead of at the beg.
sue- knitting heretic