Plastic storage box (the bigger the better)
Low Watt soldering iron
Wire Mesh (Be sure the gaps aren't too wide, mine was half-inch x half-inch)
Wire cutters (or small pliers)
Hobby knife (mine is a very thin knife I got from games workshop but you can use a stanley knife and a drill to drill holes in the wood)
Hardwood (Optional if you wish to make extra levels, similar can be used, even your recycled plastic that's been cut from the sides)
Wooden Dowels (optional)
Wood glue (optional: Non-toxic)
Finish (optional: I used "Mod Podge" non-toxic)
Cage front (optional: literally searched "cage front" on Google and bought one that was suitable. Recycling from old cages is fine too. Bar size half-inch apart. Of course you can skip this part and just open your cage from the top, but if you want to stack them like me then you'll need a door at the front)
A note about wood
As I created this guide with hamsters in mind, you may have to look up information for your specific species of pet.
There are certain sources that say do not use cedar or pine or any sort of wood that contains glue such as MDF or plywood. It is said that these can sometimes contain things that could irritate your hamsters lungs and cause reactions.
In my opinion, it is really about seeing how your hamster reacts to these woods. I have used pine wood shavings for many years without incident. The wood used in this guide is hardwood which is suggested to be safe for hamsters.
Choose your wood carefully and keep an eye on your animal for a while :)