Administering tests
Administering a code test is as simple as sending candidates an e-mail with the instructions and any necessary additional resources they will need. Be clear about the time frame in which you expect an answer; a week is usually plenty of time.
Code tests can be administered at a variety of points during the hiring process, and determining when that is depends on what's convenient for your company and what makes sense for the individual position. In general, for lower-level positions where you'll receive more resumes, it makes sense to administer code tests earlier in the process as a quick way to narrow the field. If you have fewer interviewing resources, it also makes sense to do code tests before the phone screening.
Note
Make a note
You may also choose to administer a "live" code test during a phone screening if you want to watch candidates work in real time using a collaborative editing tool such as Google Docs. Live tests can cut down the evaluation time per candidate...