Gather your coasters together and plan out your shape on a table. For mine, I made a simple rectangular frame.
When you have your shape planned, the rest is very simple. You take the Sticky Tac and attach a few pieces to the back of each coaster. To make it flatter, I just pushed it down with the edge of my fingernail a few times.
After you've placed the Sticky Tac on the back, put your coasters on the wall in your desired shape. To attach them to the wall I used a very primitive method of banging my fist against the coaster to make it stick, but I'm sure there's something more effective out there. (I have a habit of wanting to make something and making it right then whether I have what I need or not.)
When everything is in place, add your thumb tacks.
From here you have many options. You can put a chalk board or dry erase board in the center for a more functional use, or, like I have done, you can put various pictures and other decorative things in the center.
I didn't use a leveler or anything to make mine precise and perfectly aligned, I just tried to figure it out by sight. If you want it to be perfect, however, a leveler and a ruler would be helpful.
I also added some things around it to make it a little more decorative.
The only thing I would warn about with this project is if you live in a very old house (I live on the bottom floor of a house built in the '20s) and there are many layers of paint, it's very easy for the paint to come off when removing the coasters even if the Sticky Tac is supposed to prevent this. I've done this in a newer house and never had that problem, however.