The way I learned to make collars was to measure and add all four sides together then add 2.5 times the thickness of the collar for the overall length of the collar.
Next put opposit bevels on each end of the flat collar. Hammer, break or releave the edges that wil remain exposed or the out side of the collar. Chase or chissle a groove or grooves down the center of the collar for a decorative affect.
Now you're ready to use the collaring Die. Make a die the same size as the piece you are colloring. Form the first bend of the Staple in the vise, place the forming die in the fold and fold the second leg of the staple. These two bends can be done cold but sometimes you'l get a crack form on the outside corner, sometime I have to use mild heat (red to mid orange) with a torch to keep from cracking. Now you have a proper fitting collar.
When installing the colar, make sure you are on a solid surface (nice steel layout table or an anvil if you have a good way of supporting the rest of the project), with a torch, heat one side of the collar to a bright orange toward yellow. Start hammering on the side then work your way around the edge sharply bending the leg of the staple. repeat for the other leg of the staple and you will find the bevelled edges you started the collar off with will oveerlay each other for a nice closure. Quit hammering on the collar.
Later after all of your installed collars had coold naturally, check them to see if they are tight. Tap of the work somewhere lightly. If you hear a buzing rattle sound, one or moore of your collars are lose. To tighten a loose collar, use a pair of vise grips and clamp across the opening side of the loose collar. Clamp it tight! Use a torch and heat the long flat sid of the collar (Top of the original Staple) the steel will expand, however the vise grip will prevent it from moving and forces the metal to Upset within itself and whne cooling it shrinks therefore tighteniong it self back up.
Hear is a photo of a sample peice I made for customers and it shows several types of joints, collars, rivets scroll, etc. there are 21 different processes in this piece. Enjoy!