There is only 1 thing I have added to this project and that's underlay / lining support, which makes mats firmer and more durable. I applied iron on thick vliseline. You can buy it in local shop with fabrics or some bigger craft shops.
Step by step:
1. Cut squares as per pattern using 'stanley knife' or use scissors. I've found scissors to work better with soft felt, however stanley knife is more accurate. Before cutting fasten each square with a pin.
2. Begin insert corner pieces through the slots in the middle
3. Connect as many squares as you want. You can create small coaster, table mats or even carpet! That will be my next project :)
5. Woohoo, all done! :)