Texas Tomato Cage 55 gallon drum trick
When assembling two part Texas Tomato Cages often they will try to fold up on you while you are trying to insert the legs of the top part into the bottom part. If you happen to have a 55 gallon drum laying around (all gardeners should have several drums for storing rain water or mixing soluble fertilizer in) use the 55 gallon drum as a form. It is exactly the right size to fit inside a cage and hold it upright while you are trying to connect the sections.
Some other tips:
The quality control on the cages isn't that great and you will have to find the preferred orientation for inserting the top section into the bottom. One way will often line up the legs better than another orientation. In extreme cases you might have to play musical chairs until you find sections that match up without forcing.
Another tip, the only thing keeping the cages from folding up once assembled is the legs being stuck in the dirt. Sometimes as the season progresses the dirt may loosen up and the cage may start to lean or even try to collapse. So bang a long stake or piece of rebar into the ground along one side of the cage to keep it straight.
A third tip, don't wait too long to place the cages over the tomato plants, as sometimes the vines may get too stiff and break when you try to train them inside the cage. And keep up with the training every few days as the vines try to grow out of the cage.