I have to make a cake to make and the client is requesting their initials be place on top of the cake standing up. I have a template of which I will use to cut the letters, but what do I attach to the back of them to make them stand up? What do I do when the letters/numbers have a circular motion, (i.e. the number 50?)

You will need to you gumpaste to make the letters. I use fondant mixed with tylose. The would need to be made in advanced so that the numbers have enough time to dry. Roll out the fondant so that it is fairly thick. This is so you can insert toothpicks into the bottom of the numbers which will be inserted into the cake so that the letters can stand. Not too thick cause then they will take a long time to dry. After the numbers/letters are cut, wet the toothpick and slowly insert then in the bottom of the number/letter. I usually use two so that it balances nicely. Leave them to dry on a flat non-stick surface. To cut an 'O' you can cut out a circle and then cut out another circle in the center of that circle. I leave the circle to dry a bit so that when I cut out the center it doesn't get distorted. I did this recently for a 70th birthday cake. You can look in my pictures.

cocktail sticks, lollipop sticks, skewers stuck to the back. use chocolate or ri to glue them on, let dry and you will be good to go! you can make two of the shapes so you can stick one on the back, to hide the support. or if you make the shape cuncky or sausage like you can insert the support.
for circular ones, you can put the supports at an angle. try making a wide base (so the bottom of the curve is quite long) as you can then use vertical supports.
xx

Now, I know this is not gumpaste but I think this is an awesome tutorial anyway.
http://cakecentral.com/cake-decorating-ftopict-654089.html