On the Spectrum paper and mount board thread, I have given some advice about what you will need to make the mounts.viewtopic.php?f=2&t=3031&sid=42bd295109edc67a21022abdca2e7ec9#p14016
If you are using a third party to print your images, there is a lot of advice on this thread:viewtopic.php?f=2&t=1346&hilit=printing
Paper choice is something else to consider. Some images work better on gloss or lustre papers, whereas others are more suited to matt. The difference will be less obvious under glass but the judge will be making their assessment when the prints are unframed.
If you use a window mount, attach the print to the back with masking tape along the top edge only. That way, the print is free to move within the mount and will avoid any issues due to it not lying flat.
Window mounts look better if the aperture is positioned asymmetrically. The border at the bottom of the window should be around 15% wider than at the top. The sides should be symmetrical. Unless you are looking to create an effect with the contrast, mount board which has a core which is the same colour as the board or white will have a better appearance than black. (When the window is cut, the core is visible in the edges of the aperture.)
You will need to think whether you want to cut the window so that there is a white border between the edge of the image and the window. (If I do that, I usually add a very narrow black stroke border on the print. It is not large enough to be obvious, but I want to demarcate the edge of the image at a subliminal level.) Of course, having such a margin around the image does make it more difficult to position the print in the window to ensure that the gap is even along all the edges. You need to decide whether it adds to the overall impression.
Some form of backing will help stiffen the mount, which can warp over time, and protect the back of the print. Ensure that there is nothing sticky on the outside of the mount. The board is delicate and easily damaged so you do not want your print attaching itself to another one.
The alternative to a window mount is to stick the print directly to the board. I have never done that, so cannot advise. My assumption is that glue has to be sprayed on the back of the print which has to be placed on the board in the correct position first time.
Remember that the mount contributes to the impact which the print makes. Generally, it is best to avoid strong colours unless they complement the colours in the image. Judges rarely comment on mounts which have neutral colouring such as off white. Black can also work, depending on the image. Anything else will invariably invoke a remark which is almost always negative.
I use a Longridge mount cutter to cut mounts to size and create the window with a bevelled edge. It works well but is relatively expensive, although you are unlikely ever to have to buy another. They can be purchased at a discount at The Photography Show which opens at the Birmingham NEC this weekend.
Other members are using less costly Logan mount cutters and presumably can advise.
I think I have covered the main points but feel free to ask if anything is unclear or you have other questions. Other are free to add their own comments.