You are going to use:
three large sheets of plastic craft canvas
leather, pleather, or whatever you are going to cover it with
fabric to back it with and cover the mess of threads
1" thick foam
1"wide leather belt long enough to go around your waist with your bodysuit on
pliers
really sharp scissors
white marking pencil (for the leather)
permanent marker (to use on the foam and plastic)
sharp leather sewing needle
heavy polyester or heavy nylon upholstery thread
2" piece of velcro if you are going to use it
bandaids

I used a modified, scaled pattern from the Vader Maker templates. Because female body dynamics are different, I had to change the curves to allow for how my legs swing slightly inwards. Whatever pattern you use, cut it out in cardboard first, try it on, and adjust the fit and shape accordingly. Comfort and ease of movement are important here.

Once you are happy with your pattern, trace it onto two sheets of the plastic canvas. This is going to act as the stiff backing for the codpiece, but it is also flexible enough that you can sit down without being uncomfortable.

Using the upholstery thread, sew the two pieces of canvas together with the upholstery thread down the center and around the edges. Set your stitches in about 1/2" from the edge. Double up the thread as this is going to be what you use to anchor the leather in place. Do not use cotton thread or light polyester sewing thread for this as when you start pulling on it, these will both break!

Take the third sheet of plastic canvas and cut two pieces out of it which are the same shape and size as the central indent (squared off or oval depending on what you prefer). Sew these together with upholstery thread, 1/2" in from the outer edges, like you did with the larger canvas pieces.

Cut the leather codpiece cover at least 2" larger than the pattern. You will also need a leather strip 2" wide by the length around the center inset (about 20", but measure to be sure and give yourself 2" extra). The inset cover should be cut 1 1/2" larger than the pattern. The excess will be trimmed away later.

There are two ways to deal with the top of the codpiece - leave it as is - 1" thick all over - or create a ‘shelf’ that the belt and belt boxes sit on top of. Since I find that the belt boxes stick out too much on me, I’ve opted to not have the foam padding along the top 1 1/2" of the codpiece. Whichever choice you decide to use, you will need to cut the foam accordingly. Cut the inset area out of the foam and set that bit aside with the plastic mesh and leather you cut in the same shape (oval or rectangular). You will be using those later.

Take the leather and cut out the inset area, leaving about 1/4" seam allowance. Fold the strip of leather back on itself along one side, creating a smooth upper edge. You will be sewing through three layers of leather, so use the pliers to pull the needle through and save wear and tear on your fingers. Sew your leather strip to the codpiece leather along the inset seam. Where you will be stitching is shown in red in the diagram to the left. You will have a seam that you will have to hide where the two ends of the strip meet. In a rectangular inset, you can put the seam in a corner, with an oval one, it’s harder to conceal.

First you will need to reduce the thickness of the foam by about 1/4-3/8". If you don’t the inset will be too thick. Take the leather, foam, and plastic canvas parts of the inset you set aside earlier and stack them. You will be covering the foam with the leather and anchoring it to the plastic canvas. Trim the leather as needed so that when stretched it meets the guideline thread you sewed on the plastic earlier. I sew one side first, then trim the opposite one so that I don’t cut too much off. Do not over stretch the leather. It should lay smoothly over the foam, but not squish the foam excessively. To smooth the corners, trim away any excess leather. Note that you are not sewing the leather THROUGH the plastic, but anchoring it to the upholstery thread. See pic of how to do this the left.

Lay the leather codpiece cover overtop of the foam and plastic canvas. The inset will be framed by the seam between the codpiece cover and the leather strip. It should sit just at the top of the cut foam and with the strip folded down and inwards. Inside the inset area, where the leather strip meets the canvas, you will be sewing the leather strip through and to the plastic canvas. Sew OUTSIDE the line you have marked on the canvas for the inset. This is so that there is enough space for the inset piece to fit into. Sew the short ends in place first. Do not stretch the leather strip inwards by too much or you will have trouble later. Stuff the covered inset into place. You may have to work at this as the fit will be tight. Pull the inset out and tack the two long sides of the leather strip to the plastic canvas. The inset will have moved these to where there will be room for it to fit. Replace the inset. It should stay put without requiring any stitching, but I use a few stitches at the top, bottom and sides to keep mine in place.

Now comes the big job - stretching and sewing the leather to the plastic canvas around the edges. If you are making a codpiece, with a ‘shelf’ like I am, you will do this slightly differently. I will give the instructions for that first, then for one without.

For a codpiece with a ‘shelf’:

Sew the leather along the top of the foam first. You will be sewing through the leather and canvas mesh. Don’t pull too much on the leather. It should lay flat and smooth over the foam. Do not sew the leather along the topmost edge yet. Leave that open.

Next, attach the leather along the curved edges of the codpiece using the same technique as for the center inset - loop the thread through the guideline stitches you had sewn through the plastic canvas. Start at the center bottom and follow the curves upwards. Trim the excess leather as you work your way along. Pull the leather so that it is smooth and flat, but don’t make it too tight. If you are using a piece of velcro to stick your belt with boxes and codpiece together, you need to sew it to the leather now.

What you do next depends on how you are going to strap the codpiece on. If you are going to rivet/sew a belt to the sides of it, your next step is to anchor the leather in place along the straight sides and top like you did along the curved edges. After that, cover the back of the codpiece with your fabric to hide the plastic canvas, and attach your straps at the sides.

If you are running a belt along the top of the codpiece like I do, you have a few extra steps to follow. First, cut the leather along a line extending out from the stitches at the top of the shelf. Stop your cut at the plastic canvas. Fold the flap over below the line of stitches and sew the leather on the straight sides in place.

Next, take the belt and lay it along the top of the codpice, above the ‘shelf’. It will sit between the leather and the plastic canvas. Pull the leather tight over the belt and anchor the leather in place along the top.

The edges where the belt leaves the codpiece can be trimmed even with the plastic or turned under to give a smoother finished look. Cover up the plastic canvas on the back with your fabric and you are done!

For a codpiece without a ‘shelf’:

If you want a velcro strip, sew that in place first. Then sew the leather along the top, anchoring it to the guide thread and working from the center outwards. Repeat this with the lower curved edges, again going from the center out to the sides. The straight sides are the last section to do. Cover the back with fabric to hide the canvas and attach the straps. If you are planning to use a belt, you will have to make a sleeve of some sort for it to run through. I haven’t tried making a codpiece with this type of design yet, which is why the illustration is of a finished codpiece with a 'shelf' on it.