From my work place I am getting a constant supply of two to four four gallon square containers every week. As these are free and easy to get, I will continually ask for these containers for the next year or so as I plan on increasing the size of my garden until I run out of space. Here are the steps I take to convert these containers into a SWC.

Here are two of my 4 gallon containers plus a juice bottle, my starting components.
This is a 2L rectangular-based juice container. I pick rectangular because it will be easier to cut a rectangle out of the bottom of the upper container than it will be to cut a circle. I have three more of these juice containers waiting for future SWCs, but once those run out I will have to resort to other sources.
So the first thing I did was cut a hole through the bottom of one of the containers making sure that the hole I cut is smaller than the rectangle of the juice container. I just used a utility knife to do this.
I cut the small hole so that I could see what the depth was. By putting the container with the hole into the other untouched container I could now measure how deep the reservoir is. Conveniently it is exactly three inches.
From the top of the juice container I measured down three inches and cut it off. By putting this upside in the base it will allow for more water in the reservoir.
Using the top part of the juice container that I had cut in the previous step, I traced an outline in the bottom to be cut out.
I do not want too much earth to get from this upside down top into the reservoir, so I used packing tape over what used to be the cover. Using a small drill bit I then drilled four holes through the neck, and using the knife I put slits all around the sides. Hopefully this will be enough water infiltration to allow the soil that rests in this wick part of the container to properly wick the water up to the roots of whatever will be growing here.
A key component of a SWC is the overflow hole, on two sides I drilled a hole two and three quarter inches from the bottom. This allows for a quarter inch gap between the top of the water level and the bottom of the upper container.
So here is the photo with the wick container in the bottom of the SWC.
Next I have read that putting some holes in the bottom of the upper level can help for a few reasons - allow for drainage if the soil ever becomes too overloaded due to rain or overwatering from above, and to allow some roots (if they get that deep into the soil) to pass through the holes and tap into the reservoir.
Here's the bottom side of the bottom container, ready for a new hole to be drilled.
I purchased a pipe adapter that has threads on one side and a pressure mounting on the other, a 3/4" hole drill bit was used to drill this hole.
I also bought some multi-purpose silicon sealant since it isn't clear that the screw in part of the adapter will have a perfectly water proof fit. I coated the threads in this silicon sealant.
Then finally it was a matter of screwing it in. After I screwed it in I added a bit more sealant on the outside all around the joint, just to be sure.
This is the configuration of the six containers plus one reservoir. All of the PVC transition pieces are in place, the left two are joined together while the three along the top and three along the bottom are also joined, while the diamond water reservoir is connected to the end of each row of three. Close ups are in the two photos below.
The two pictures below show the tubes connected and the water test for leaks.
It was a bit tricky, but with help from my three year old daughter I moved the seven connected containers over to the garden area of the lawn. In doing so one of the silcon sealed joints wiggled loose so a repair job was in order. To the right you see the setup in its correct place, I have put potting mix into five of the containers and the sixth has a plastic bag covering where I have put more silicon on the loose joint.
Once the broken seal was sealed again it took a few weeks before I bought the strawberry plant that was going in it, I finally did and here is the last photo of the build the SWC page.