I use it. I find it pretty easy to use.
Basically, you start out with 10? (I forget) slots to fill up with Cartwheel offers, which are basically just coupons. Typically they're 5%, but for certain items they can reach 50%. You have to add an offer to your Cartwheel and then present your Cartwheel code at the register at checkout. If you don't have a phone, you can print out the barcode. I actually taped it to the back of my REDCard because I don't like struggling with my phone at the register, especially if I'm already struggling to pull out paper coupons.
You can use the Cartwheel app in store to scan items and see if there is a Cartwheel offer available.
There are three approaches you could take
1) Use the Cartwheel app on your phone to scan items while you're selecting which one to buy, so you know which item is cheapest after the Cartwheel discount, if applicable. This will maximize your savings but takes more effort than the next option
2) Just pick up everything you were normally going to buy, and then scan everything with the Cartwheel app to see if there are any offers on anything you added to your cart, and then add the offers to your Cartwheel.
3) Hope that Cartwheel will autoadd the offers for you. I swear to God I didn't add a couple offers to my Cartwheel that I actually received at the register. I think it auto-added them or something. I wouldn't count on this happening though.
It sounds like #2 is probably the best fit for you, though you could just go for option #3 and see if that works.