The saddle cloth type is the easiest to make. At it's most minimal a simple western saddle pad with decorated edges will do. This type can be extended almost to ground length and decorated with dags on the ends. It can cover more of the horse than a regular pad, but is rarely big enough to be mistaken for the "full coverage" type. It always looks like a strip of cloth going over the horse, and that is probably just exactly what it is. One 45" wide by 3 yard piece of fabric makes a good one. Just measure across your horse's back and down each side to find how long to make it. Stopping the ends at about hock level is good for looks and high enough not to entrap your horse's feet. You can dag the ends or put fringe on it. If you want a fancy look you can bead or applique designs and put metal danglies on the edges. Refer to period paintings for ideas.
This is just a simple length of cloth with dagging or scallops on the ends.
2. full coverage
The full coverage type is the next easiest to make, as the pieces can be completely flat and just tie or hook on. The simplest style requires just a couple measurements and no fittings. It is the closest thing to "one size fits all" that it is posible to get in a full coverage trapping.
This style is in two pieces, one that covers the horse's rump and attaches behind the saddle and one that goes around the horse's neck, sometimes covering the pommel of the saddle.
The rear piece is a slightly stretched half circle and the front piece is most of two quarter circles.
Other alternatives for full coverage bards are to purchase a sheet and redecorate it, or to get a sheet/blanket and hood pattern from a company like Suitability and make them up in your own colors. These work well for short bards that just cover the body.
I don't recommend making long ones that come past the knees or hocks as many horses trip on them. You will have to decide for yourself.
3. open or strap
Strap or open bards can be decorated breeching and breastcollars or much more elaborate. the fancy ones with a "spiderweb" of straps on the horse's rump requires the most fitting of all. Decorated breeching and breastcollars are very economical to make. Just cover a ready made beast collar with fabric of your choice and trim to your hearts content.