It depends!

All sidesaddles are not the same. Most, however, use the same basic stuff.

If your sidesaddle is English:

It probably has three girths.

1)Main girth - same as your astride english saddle. Use the first two billets if your saddle wants to slide forward, the last two if it wants to slide back, the first and third if it stays where it should or you just don't know, yet. Tighten as you normally would.

2)Balance strap - goes from a billet on the left front (probably on the point) to a billet on the back right. Should be pretty snug but not so tight as the main girth.

3)Overgirth - holds the flaps down. Just needs fasten so it is not loose.

There are exceptions to this:

Some sidesaddles have a buckle on the left point instead of a billet. You will need a different balance strap than other riders. Yours will look much like a stirrup leather with a buckle on one end instead of both.

Some sidesaddles have a spring flap on the offside instead of an overgirth. You'll only have a main girth and balance strap.

Some older sidesaddles may not have a balance strap at all and may or may not have an overgirth. They aren't meant for jumping.

Some older sidesaddles may be missing a strap through no fault of their own. This can be fixed by a competent saddler.

Some older sidesaddles used a specially made girth that had the balance girth sewn to one end. These saddles won't have a nearside balance billet. My Beck-Morrow is like that. Because I don't have specially made girth, I buckle my balance strap to the first billet, just below the main girth buckle.

Your saddle may be what is known as a Piezokrapp. This isn't a brand. It is a term used to refer to any saddle of questionable origin, probably Paki or Indian made. Many of these have straps in places they shouldn't or don't have them where they should. Some have a balance girth on the near side. I s uggest that you remove it. However this is probably the least of your worries with these saddles.

If your saddle is western:

Whole new set of possibilities, here. First, as with the English, there is a main cinch. Fasten that one so it is snug. Then you may have either a flank cinch or a balance strap. A flank cinch will need to be attached to the main cinch to prevent it slipping back. It will be cinched as tight or nearly as tight as the main girth, unlike the flank cinch on your astride saddle. A balance strap will be fastened as with the English sidesaddle.

Some western saddles have a short balance strap that fastens to the offside cinch buckle. All balance straps need to be snug.

HINT: Put the half-breed on the nearside and the latigo strap on the offside. With this setup, you can tighten your saddle while mounted without raising your apron!

You may or may not have an overgirth on a western sidesaddle. Some may have only the main cinch.

If it has a flank cinch, but no balance girth, you can adapt a bit. First possibility is to untie the near side of the flank cinch, and move it to the front rigging ring (bearing in mind that you may want to move any half-breed straps to the near side). Fasten it as usual on the off side, with the flank-cinch-turned-balance-girth going diagonally under the horse's belly, over the front cinch. It will be snug, like a regular cinch.

To keep the balance girth from shifting out of position, you will need to add something to the front cinch. English riders have a small strap, or sewn-on keeper, on the main girth to do this. What works easiest with a western cinch is to buckle a small strap, or tie a stout thong, between the 2 small D's found on the middle of many western cinches. (There's an illustration of this in "The Western Side-Saddle.") If you neglect to arrange something like this, the balance girth can slip back, off the cinch, and suddenly a bit of belly skin gets pinched between the two.

You can create a "half balance girth" that is stitched to the main girth, rather than going entirely under the horse. But it's called a quarter strap. Remove the flank cinch and the nearside latigo or half-breed strap. Run the offside latigo diagonally over to the offside cinch ring and tie it. It isn't as secure as a full balance girth, but will do fine for rail work and anything not too strenuous.

Many show saddles nowadays (including some modern side-saddles) are made with in-skirt rigging. This IS NOT suitable for side-saddles. It is simply not as secure as on-tree rigging. The other problem with in-skirt rigging is that you probably lose the option of changing the flank cinch to either form of balance girth because the ring or slot is probably too low to make this a good option.

If your saddle is an antique:

First make sure it is safe! Tree is solid and billets are pliable and not dried out. Best to just plan on replacing them from the beginning.

It could have anywhere from one to four billets. If it has a Y-shaped setup on the offside, this is your balance strap and overgirth all in one.

If the saddle fits you and your horse, do yourself a favor and get the billets replaced with something that will take a ready-made girth or cinch. You'll drive yourself crazy trying to have authentic girths made to go with the saddle.