The fenders and leathers are next. This process is more than just simply cutting out the parts and assembling them. The stirrup leathers must be stretched, the process of this is to wet the leathers, then glass or rub them, then spike them on a board and allow them to dry completely. This will help minimize the stretch in the leathers when you ride the saddle. If this process is not done somewhere down the road the leathers may stretch some from one side to the other creating uneven stirrups. This will help solve that problem before it is created. Another thing to take in account is your leather selection, Stirrup leathers are taken from the very best part of the hide, you should also select the best of the two leathers for the near side since it will take most of the abuse with you getting on and off your horse.
Once the leathers are properly stretched, then you can assemble the fenders and leathers. The the fenders and leathers go on the twisting and stretching devise. This will again help with taking some of the stretch out of the fenders and leathers, it also shapes the fenders and leathers so the stirrup is always hanging in the proper position. If you are building a custom saddle and you do not shape the fenders and leathers so the stirrups hang proper...shame on you!!!
As a custom saddle maker we are paid and expected to do the little extras like this on a saddle. There is much more to building a saddle than just the simple assembly.