Eric has already posted the link to my company website that talks about mounting but I will make a few extra comments. I will generally just mount the print to the mat board in just two points at the top. This will keep the image centered in the mat opening. If you are not going to place the mat and back board in a frame then I would mount it to the back board. If you do this you will most likely want to hinge the mat to the back board with some linen hinging tape such as Lineco:
A good product to hinge the print also from Lineco is their hinging tissue. It is not as archival as Japanese rice paper and water based starch, but it is much better than Scotch acid free tape and a lot easier to use:
The key is to leave the print free to expand and contract because it will do so at a different rate than the mat and back board. The mat will keep it flat unless you are in a very humid area. In that case you will have some wave in the print. The only way around this is to dry mount it. Dry mounting is not archival. Museums will just tolerate the wave on their prints so it is considered perfectly fine by high end collectors, but will be frowned on by someone with no framing knowledge that is just looking at prints in at an artshow.
You can cut your own mats but it takes a lot of practice and unless you get real good or have a real good cutter it will be obviously be hand cut. This may or may not be an issue depending on whether you are trying to sell the.
I am a photographer myself and I hated the lack of mats and frames for the 3/2 aspect ratio and that is what led me to found Frame Destination, Inc. We carry museum quality Nielsen and Bainbridge framing products. 8x12, 10x15, 11x17 and 13x19 are standard sizes for us and we also do custom since many of our customers are displaying their work in galleries and need signature borders or prefer bottom weighting.
Feel free to contact me with framing questions. I am always happy to help other photographers even if we cant help you with our products.
Frame Destination, Inc.