Measure the size of the area you'd like to cover with tatami.
Purchase interlocking activity rubber mats from your local fitness store or children's toy store. These mats are usually about 2 feet by 2 feet, and you can interlock four of them together to create a 16-square-foot mat area. If the dimensions of the rubber mats don't correspond with the dimensions of your room, buy slightly more rubber mats than you need; you can always cut the mats to fit your specifications.
Purchase straw mats to cover your interlocking activity rubber mats from a home decoration store. Someone from the store may be able to cut the mat to your specified size or you may need to purchase more than one mat.
Cut the activity rubber mats to suit the dimensions of the area and the straw mats.
Set the rubber mats on the floor and put the straw mats over them. The straw mats will stay in place through friction with the rubber mats. The weight of the straw mats will also help keep it steady.