That depends on where you live! If the DVLA have asked for you to have this test, they should have also sent you a letter telling you which Optician will do the test.
The test has to be done by a DVLA approved optician - if you have received the information from them, you'll need to contact the Medical Branch in Swansea to find out what's required.
The test is thorough, but I scored 100% - and that was last year after copious lasering in both eyes, whereas I found the Eastermanns machine quite vague and difficult at times.