I am working on a raglan rib sweater. The pattern stated to work on the pat til it measures 11inches.?

I am stuck where it tells me to shape the raglan (back). It says this: keeping cont. of pat, cast off 3 sts beg next 2 rows. 87 sts.(Large size). should I start to cast off after I do two rows or cast off the last row of the 11 inches that I did? I am confused. .