There are some great tutorials out there, I used this tutorial and adjusted a few things on it but Tip Nut has some great beach tote turorials including one for a totable towel which I may break down and make as well.
I made 2 bags for the beach, one may end up being a gift later if I decide not to use it but I loved both the pink and blue fabric so made two bags. The blue one is a bit bigger, has more pockets inside and out (including the ruffle on the back) and has stiff interfacing to make the sides and bottom rigid. The pink one has wider handles and only has the stiffening liner in the bottom as well as less pockets.
I added a cell phone pocket to the bag too.
I used my phone as a template and added 2 inches to the width and length and cut out a piece of heavy duty plastic, you can get this by the yard at your fabric store or you can use some of those plastic bags sheets and bedding come in.
Sew along the bottom 3 sides after you position it in the spot you want it. You can see it in the top left corner of the bag below. It will also work great for ipods/MP3 players
Inside of the blue bag-you can see all of the pockets on the inside sides.
Back ruffle is a little different on the pink bag and does not have the pocket feature.