The plates that adorn the balteus are generally described as either narrow or wide.  The narrow plates are the older style, the narrow belts often being worn in pairs.  These plates are either cast brass or cut from sheet, and frequently tinned or silvered, and inlaid with niello (a black silver sulfide compound).  They range in width from 1" to 1-1/2", and in length from 2" to 2-1/2".  Many narrow plates are plain, though they are still often tinned.   Some have pegs on the back to serve as rivets (rivets can be soldered on instead), or are just riveted normally with domed or flat-headed copper rivets.  On many cast plates the rivet heads are "countersunk" and filed flush to be nearly invisible.  Some are tinned but with an untinned brass border.  Another example has a punch-work design of a vine with leaves, very simple to do with a hammer and nail.

Wide plates, measuring 1-1/2" to 2" wide by 1-3/4" to 2-1/2" long, are generally stamped from thin brass sheet (.010"), and are also often tinned or silvered.  Frequently the ends are rolled, sometimes with ball-headed pins inserted in the resulting tubes.  Some original plates from Germany are tinned and have concentric rings scribed on so that the brass shows through.  If the ends of the belt plates are rolled, cut notches in one plate end with a file to mate with the buckle, and likewise for the frogs.  

The plates are riveted to a leather belt no wider than the plates themselves, with domed or flat-headed rivets.  Round or square washers may be used ("square" ones being irregularly cut from scrap brass).  The leather is 3- to 6-ounce and may be left plain, or dyed red, black, or another color.  In any case it should be well-coated with neatsfoot oil, beeswax, or Sno-Seal (which is beeswax with an emulsifier).  If you use an 8-ounce belt blank you may have to “skive” the running end to get the belt end through the buckle.  The buckle is most often seen on the right end of the belt, while the free end is narrowed to fit it.  Be sure to position the dagger frogs far enough apart to accept your pugio scabbard, about 6" to 7" between the centers of the discs.  Be sure to make the belt the right length to go around you while you are wearing only your tunic so that it is not too loose when you are unarmored.

The apron, misleadingly called a sporran or groin-guard, was a decorative item derived from split belt ends.  It has from 4 to 8 leather strips 3/4" to 1" wide by c. 10" to 12" long.  The leather can be thinner than that for the belt, anywhere from 2- to 6-ounce, but if it's any heavier it will not hang properly.  Wide belts characteristically have 4 of the wider strips, about an inch wide each, while a larger number of narrow strips are usually seen only on narrow belts.  Each strip ends in a dangling terminal, and has up to 16 disc-shaped studs.  Usually these are cast brass (and occasionally inlaid), with a peg on the back to serve as a rivet.   You can solder rivets to the backs of discs, as may actually have been done with some domed Roman studs.  Studs can also be cut from sheet brass and secured with a rivet through the center.  The studs can be placed to form horizontal rows, or staggered (although the terminals all hang at the same height), or even spaced up to 2" apart.

The apron can be 12" long or longer, but always ends above the tunic hem.  A belt can certainly be made without an apron (in fact centurions' belts did not apparently have them), and it is easy enough to add an apron long after the rest of the belt is finished.  

When worn, the buckle is usually on the right side so that the belt tongue points back past your right hip.  The apron can be right next to the narrow end, and if it hangs over the belt you can hide your worst plates under it!  Then will come 2 or 3 more plates followed by the gap between the pugio frogs.  Lay all the pieces out on a table, and once you are satisfied with the arrangement, number all the plates and mark their holes on the leather.  If you make the belt's tongue extra long, you can loan it to larger friends when necessary.  There is at least one relief showing a long tongue hanging down next to the apron (with the buckle at the left end).  

Tinning can be done with instant solder paste, a gray goo of powdered tin and flux in a tube.  (Oatey brand seems to work better than Kester.)  Spread it on, heat carefully from below, and when it melts give a quick wipe with a slightly damp smooth rag.  The excess solder will be wiped off and splatter, beware!  Be sure to tin the plates before you stamp them, it's much easier and the tin won't fill in the details.  Stamping or punching will cover some of the irregularities in the tinning.