I had to submit a portfolio as an interior design student, and though it’s not exactly the same as your situation I’m betting there are some universal tips you can follow (since it’s all design-related :) ). First of all, whatever specifications they give you (and if you don’t have them, find them before you start anything—call them if you have to and speak with a live person who actually knows what they’re talking about, lol)...make sure you read them all and follow them to a tee. You don’t want to unnecessarily lose out on vital points. You’ll want to bang your head against a wall if you realize you didn’t do something as simple as labeling the exterior of the portfolio the way they wanted.
Secondly, only put your best work in your portfolio. They don’t really care to see how you’ve progressed from your first project to where you are now. Progression is not the point. They want to see where you are now and how great your work is. So don’t be tempted to put any so-so pieces in just to “fill” your portfolio. It isn’t worth it. Quality is preferred over quantity in this case.
Thirdly, once you’ve decided what pieces you want to put into your portfolio, I suggest you lay them out visually (on a computer or with low-quality mock-ups, whatever works) and decide on what order you’d like to place them in. Do you want to begin with your strongest piece and end with your second strongest with the rest in the middle, or some other way? Will you organize it by project, theme, or some other category? Will you use actual page numbers (if that’s left as an open option, not a requirement). These are the types of things you’ll want to be thinking about.
Fourthly, you want consistency, and there are several ways of going about achieving this. The use of color, graphic elements, and the type of header or footer (or both) you’d use on each page are all some things to consider. The layout of certain elements of your projects/works is also important if you are showing more than one image per work. It should be consistent throughout your portfolio.
In addition to layout, you might want to consider the hierarchy of your presentation. Using differing text sizes for titles, descriptions, etc. can help a lot to do this. Some of my (interior design) professors were very picky about text sizes. They generally encouraged me not to use anything larger than a size 14–16 font if I could help it (but you also don’t want to go too small), but when you are using text just remember that your work is what’s more important and that the text should not overpower it in any way. (As o0 said above, talking with a graphic design student/professor can give you more direciton in this area, as they are more the experts when it comes to the use of fonts and whatnot.)
And lastly, this may or may not concern you, but if there is any written portion required that will be considered as part of your overall score, then please, please do not wait untill the last minute to do this. It will be tempting to focus all of your energy on the portfolio itself, but any written portion deserves the same amount of care, as it is (or may be) part of the portfolio itself. My written portion was the reason I didn’t get into my school’s progam (I had to transfer to another school :( ) because I did not give it as much thought as the rest of my work. Have someone, or several peopel, read over it to see if it actually addresses the topic and whether it’s a good response or not. Trust me, you’ll want to kill yourself if you find out the reason you didn’t get in was because your essay wasn’t up to par.
…now take a deeep breath.
If it sounds like a lot of things to consider and a lot of work…that’s because it is. Leave yourself plenty of time to make changes later on (meaning start early!). :) And don’t be afraid to talk to professors and other students, epscially upper classmen who’ve already gone through the process. Or even take a look at their work in previously submitted portfolios, if you can. (When you say you have to mail in your portfolio, it sounds like you are applying to an out-of-state/district college, but I’m not sure so I just suggested that last bit anyways.)
Sorry for the long post, but I hope it really helps you! Good luck!