How does one assemble a metal twin bed frame?
Answers: The only other type besides the "button" one (which be described perfectly within the previous answers) is the kind that have a metal clamp that slides onto each duo of short metal pieces. They each hold a thumbscrew that you tighten by hand after the two sides are slid close plenty to be able to fit tightly around the mattress. That capability to adjust makes them better than the button ones.
You haven't given abundant details, but typically the metal bed frame will consist of two components. Each of these have a long side and two short sides which fold up into the longer side for storage. Unfold the short sides (they are hinged) on both pieces until they (the short sides) is at a 90 point angle to the long side. You will now enjoy two components which resemble the sahpe of the letter "C". Lay out the long sides parallel to respectively other with the short pieces facing respectively other. As you overlap the matching short sides you should see within the metal frame that one side has a "button hole" resembling slot. The short side on the opposite side have a "button". Fit the button into the button hole and pull the two long sides contained by opposite directions to lock the button contained by the button hole. Do the same on the divergent end. Hopefully the will fit your metal frame arrangement.