How do I get the belt?

From the sewing table/studio. You merge the buttons to get the sewing machine (level 10), and you merge the sewing small blue kits to reach the sewing kit.

Click on the sewing kit, it will give you either thread or cloth. Click on the sewing machine (that you created from the buttons) and it will take one thread and one cloth to give you clothing items. The belt is 5th level in the merge.