There are probably as many ways to do this as there are armorers.
-Start with a convenient piece of brass shim stock. I usually use .012" (.3mm). It comes in 6" wide (150mm) rolls.
-Cut off a convenient length. I usually take about 6 or 8 inches (150-170mm)
-Anneal the brass.
-Remove the oxide from one surface, so that your solder will adhere. I usually use an 80grit greaseless compound on a sewn cotton buff. This will require a backing board to keep the, now soft, brass flat while you clean it. Take care not to catch the edges in your buffing wheel!
-Cut the brass into strips about 5/8" (15mm) wide. I use a scissors.
-Cut the strips into squares.
-Using a punch and a tin block; stamp a dome in each square of brass. Be sure that the clean surface will be the inside of the dome.
-Take 1/16" (.158mm) rosin core electrical solder, and wrap it around a 3/32" (2.5mm) rod.
-cut the solder into rings.
Note this gives a measured amount of solder which is right for my caps. You may need more or less solder.
-Remove the oxide from the heads of your rivets. I use an 80grit greaseless compound on a sewn cotton buff. I hold the rivets by their shanks with a big pin vice.
-Wipe a little zinc chloride soldering flux over each of the rivet heads. This step is optional.
-Pick up a brass dome by one of its corners, using a needle nosed pliers for which you care little.
-Put a solder ring into the dome.
-Put a rivet (head down, of course) into the dome. You will now see why I make rings of the solder!
-Hold the assembly carefully over a gentle torch flame.
-Heat until the solder appears around the rivet head. Capillary action will probably pull the rivet into an upright position. If not, gently poke at the rivet with a bit of wire until it is straight.
-As soon as the solder is melted, gently remove the assembly from the flame.
-Carefully set the assembly down to cool.
-Repeat until all your rivets are soldered.
-Take each rivet ( now with a cap) and trim the excess brass away.
-Polish the brass caps by whatever method you will.
-Electro-gild if required