It's very easy to do online.
I always double check (in the e-mails they send me) that they listed all the items I qualified for. Walgreens usually gives you a week (I think) (meaning do it before Saturday), and Rite Aid gives you a month to submit and claim.
Here is the Walgreens EasySaver link
Here is the Rite Aid Link
Oh, and if you didn't know it yet...Walgreens has a $5 off $20 coupon for Friday and Saturday of this week. You can get it here.