Welocme to KH!
Decrease at each end by k2tog or ssk or whatever decrease you used in the body of the purse. You'll have to watch the sts on the needle to make sure that you knit the purl sts and purl the knit sts so that the seed stitch pattern continues.
You switch to seed to mirror the beginning of the purse after the cast on. When you cast on, you increased the stitch number in seed stitch. Now you're decreasing in the same pattern. The opening will then have a seed stitch pattern on both sides. If you didn't increase in seed stitch at the beginning but did all the increases in stockinette, then do all the decreases in stockinette before switching to seed. One side should measure the same as the other.