A couple of websites I consulted suggested the mix of shellac, Venice turpentine and pigments to make the wax, but you can actually buy some ready-to-use seal wax in specialized stores. Some of them even sell wax with wicks, so you don't make a big mess lighting the wax stick. If you're old school, though, go for the mix and tell me how it was! If you think about it, the possibility of creating a personal wax collor is also pretty cool.
After getting you wax ready, you just burn it on top of the envelope or paper you wish to decorate and, before the wax gets cold, press your signet on top of it and remove it gently. A couple of attempts might be necessary to get everything right, but practice makes the master!