Ideally you should already have all the stuff you'll need around the house, except perhaps magnetic paper, which can be found for incredibly cheap (see below). Basically what you'll use for this project are: a hobby/exacto knife, a ruler, scissors, superglue, basing materials of your choice, and...
some excess sprue
V. Arranging the bases of the unit on the tray to get an idea of the size I'm going to need (note that I tend to use spare bases for this rather than the actual unit), I roughly trim the tray down, leaving a good amount of leeway on all sides.
There are several options for magnetizing your unit's bases, from rare earth magnets you can buy at a hobby shop just for that purpose to simply attaching small metal washers, but to keep with the cheap and easy theme, you can just cut out squares of the leftover magnetic paper and stick them to the bases. This works fine for a few games, but they do tend to peel off over time, so its better to superglue them. Just be careful with polarities, as the way the square is facing makes a surprising difference. Best to do one at a time and rank them up on the tray as you go, to make sure they don't repel each other.
I also put a strip of the leftover magnetic paper on the bottom of the movement tray once its done as I transport my army in a large covered baking sheet, like this:
One dvd case will easily make a movement tray for any standard-sized unit, but its just as easy to make a horde base by simply combining two dvd cases lengthwise, joining them using an uncut magnetic sheet.