If you have the vacuum machine you can just order the plaster and the sheets (eg sof-tray ultradent classic) and start making them.
1. Get another nurse or dentist to show you how to pour up models. This is not as easy as it may seem, if you don't have a a vibrating plate you need work hard to get rid of all the air bubbles.
2. When the plaster is dry carve off any excess with a strong knife. Fill any voids with plaster and allow to dry. Apply nail varnish or another spacer to buccal surfaces of the teeth if your dentist wishes the whitening trays to have resovoirs (some dentists insist these are needed, others do not). Carve a 1mm deep ditch around you plaster cast 1-2mm above the gum margin. This produces a tight fitting tray.
3. Heat and then suck down the sheet until it clings to the plaster teeth.
4. Pull off and trim with scissors.
Try making a set for yourself first! Have fun!