First you BO 4 sts at the beginning of the next 2 rows; that will form a 'notch' that's the underarm. Then the next RS row k3, do a decrease, work in the shaker st pattern to the end, dec and k3. To work in the st pattern, you will work the sts as they appear in your knitting. You've just decreased 5 sts so you can't begin it with the same st you did when you had all of them. Looking at your work, you should be able to see that 'this is the next st, so it begins here'.
sue- knitting heretic