Want to get into worm composting but don't want to buy a professionally made bin? This guide is for you!
Picking a Bin
As long as worms have air, water, and food, they wouldn't know the difference whether or not they were just in a big pile of organic matter on the ground, or a fancy worm bin.
The worm bin is mostly for you to keep all of your worms and the contents of the worm bin in a confined location.
I've seen people use things like old refrigerators/freezers and old bathtubs, but 99% of people doing DIY bins go for plastic totes. Usually between the 5 gallon to 50 gallon range - so most of this post will be geared towards those using plastic bins.
Make sure it is at least 6 inches deep
Make sure it has at least 1 square foot of area per pound of worms you'd like to have
You probably want it to be able to hold at least 5 gallons total, but I prefer the 10-20 gallon range.
You can have worms live in a bin thats shorter, or smaller, but you won't see much results from those bins. You won't be worm composting as much as you just have some pet worms. But if thats your thing, go for it!
Your worms need to have air. Without air they will die.
I never put ventilation holes on the side of the bin. Its not a huge deal, but I'd rather put them in the lid (if im even using a lid) so that the worms cant use those ventilation holes to escape. If you have plenty of holes in the lid it will provide all of the air that the worms/bin will need.
In, for example, a lid thats 1 foot by 3 feet, I would put about 10 1/2 inch holes drilled into it.
You can also just cut out a section of it, too.
Additionally, if you want to help keep fruit flies out, you can lay a breathable fabric/wire mesh over the bin to allow airflow, but prevent insects from finding the bin.
This has some conflicting information online.
Your bin does not need drainage holes. If you bin is leaking water out of the bottom, it is too wet.
If there is liquid coming out of the bottom it not worm tea, its worm leachate, and it is not recommended to use on plants.
Drainage holes are also annoying because worms will sometimes use them to escape the bin/go into a water collection bin, where they might die if you don't collect them and put them back into the bin.
BUT, very small drainage holes can still be good to have.
I use a 1/16" drill bit to put holes big enough for water to drip out, but too small for worms to escape, in the bottom of the bin.
Then you can place something underneath it just in case. A towel, an upside down lid, another bin - whatever.
Again - you shouldn't have water dripping out of the bottom because your worm bin should never be that wet/waterlogged, but its good to have that protection in case it does get too wet.
Stacking System, CFT, or Batch
From here you could leave your bin as-is and it would be a "batch" system. You will add food into the plastic bin until it is full and the worms have eaten everything, and then you will harvest it all and restart. Very simple.
To make harvesting worm castings easier, though, people will often modify their DIY bins further. Either making a Continuous Flow Through design, or a Stacking Tray design - which is a different approach to a CFT system.
Because a stacking tray system is more common for DIYers, I'll talk/type about that first.
Stacking Tray System
In addition to the first plastic bin that you have, you can get a second bin (preferably the same bin so the bottom of the second one can fit into the first one), and drill bigger holes into the bottom of it. Something like 20 1/4" holes.
When your first bin is full, instead of harvesting it, you now remove the top 2-3 inches of material from it, place the second bin (with all the holes in the bottom) on top of the first bin, place the top 2-3 inches back into the top bin, and continue adding food like normal into the second bin.
What will happen is the worms will migrate from the bottom bin into the top one in the pursuit of food. Once the top bin has filled up as well, most all of the worms should be in it. You can then remove the 1st bin (which should be all worm castings and only a few worms) and use the material as finished worm compost/worm castings.
When you've removed and emptied the bottom bin, you then empty out the top bin's contents into the bottom one, and then repeat the whole process.
In theory this works well, but in practice I usually find that there's still a lot of worms in the bottom tray and you have to harvest it as if it were a batch system anyway - which kind of defeats the purpose, but it still definitely does help get a lot of worms out of the bottom bin.
Continuous Flow Through DIY Bin
If you're new to worm composting I wouldn't worry about trying to make a bin like this. Get the practice/skill of worm composting down first before you put a lot of time/effort into making a CFT bin.
The concept is that you make a worm bin that has had the bottom removed and replaced with something that will keep the contents of the worm bin from falling out on their own, but will let some of it fall out when you agitate it.
That way you can continuously remove worm castings from the bottom of the bin, while adding food at the top. Harvesting will be the easiest in this set up, but the bins are difficult to make and you might as well get a professionally done one like the Urban Worm Bag.