Instructables

How do I turn a imprint of a key into a actual key?

I imprinted my key in some floral foam and it was perfect but now I want to turn it into another key

The best method is to find a matching blank at the store. Blanks are available for almost every modern mass-produced lock.

There are also key-cutting tools you can buy online or at a locksmith's supplier.
This is one example: http://www.dealextreme.com/details.dx/sku.16425
They vary, but generally the process is to measure the teeth of the original or the impression of the key and snip off pieces of the blank to match. A good, well-designed tool should be quick and easy to adjust and able to snip pieces off quickly and accurately. Ideally, it'll work for a wide variety of keys and not just one specific width of tooth.

But... you can probably fake it with basic tools. A couple small metal files and a hacksaw blade come to mind. Measure the teeth on your impression and cut the blank to the same heights and widths. Keep in mind that small mistakes are often ok on widths, but not on heights so measure twice and cut once! It's not hard, but requires a little practice to start getting keys that work. Buy a few blanks.
Of course, you can also bring your foam to a locksmith and ask there to have it done. It'll probably cost you, but a decent locksmith should be up to the task.
Of course any decent locksmith should ask you why you didn't bring in the original key... because generally this sort of thing is an attempt to get a copy of a key that shouldn't be copied, and nobody really wants to encourage that.
Naw they don't usually ask. They have every right to assume that their customers are on the level (and they usually are). Lawbreakers very rarely go to locksmiths for help.