I suggest that you decide what items you want your activity tote to hold first. By having the items already on hand, it will help you to determine what size pockets you will need for each. Try to make or buy small items that will keep a child entertained. Look for things like small books, crayons, flash cards, etc. These totes are great to keep in the car so while you are in places such as a restaurant, a child will have something to keep them busy.
Sew your two placemats together at the short ends so they make one long strip. Lay out your items on top until you have them arranged in a way that you like. Try to put similar sizes next to each other so the tote will roll up nicely.
All my pockets are cut from felt and sewn on by hand but you can use a sewing machine instead. To figure out what size you will need for each pocket, lay item on the placemat.
Lay felt on top and allow extra on each side so it covers sides of item and you have enough to sew to placemat.
Do one item at a time so you can check the placement of each as you go. The first row of items will determine the width of how the tote will roll up.
I left the tops of each pocket open. Once you begin rolling up the placemat, the items remain in the pockets. Each pocket will be a square or rectangle and you will sew the 3 sides and leave the top open on each to insert the item in.
Starting with the first pocket, pin it in place. Put the item inside to make sure it is big enough.Do this measuring for every item. Remove the item and sew the 3 sides.
If adding any trims or pictures, I suggest that you sew them to the front of the pocket before you sew the pocket onto the placemat. It's much easier this way. My grandson likes the Angry Birds so I made some of them with felt pieces and decorated his tote with them.
You can use colored hair bands to keep tote closed, if you choose. They come in all different colors and just slip one over the rolled up tote on each end. I made the closure for my tote by sewing buttons on and using hair bands as a loop.
You can make a simple drawstring bag to keep the tote in from a pillow case or piece of material.