Depending whether they're billeted or buckles. Billets attach from top (big) ring to small (bottom) ring on the same side, with the billet to the inside. Buckled ones are fitted the same but buckles towards the outside. You fasten your reins on through the leather coupling, again, billets inside, buckles out. To be correct the bit should also have a lip strap which fastens to the little rings in between the others which can be looped through the centre link on the curb chain but a lot of people don't use them.
If you can, please don't use roundings on a pelham, try to use double reins. They really aren't that hard to use, honestly, most people I teach can get used to double reins in literally about 5 mins. By using roundings you are diluting the action of the pelham. It is designed to be used with two reins, and it only works correctly if the two parts can be used independantly. If roundings are put on the bits acts as a dull pull for the horse, and very quickly in my experiance causes them to become dead in the mouth and lean even more.
I have ridden in a pelham and double over the years quite a lot. However, on my horse at the mo I am trying to keep the curb rein really soft but finding it ends up too long and baggy. If truth be known in the past I was probably actually using both reins together at even length and tension. I don't necessarily want to do this with my horse. (So probably roundings wont help anyway!!!!!) I want to ease off on the curb but an finding it hard to do this then pick up quickly enough when I need it. It's all turning into a bit of a fiddle. I intend to go back to my weymouth or plain bradoon snaffle soon, I just want to stop him pulling my arms out for a while.
The only way to have the curb rein relaxed most of the time, but have it there when you need it is to use two reins (and practice!). Using split reins will allow you to have the curb rein looser all the time, but then you loose most of the effect of using a pelham.