Go to EducationPlanner.org to find a free sample PSAT test (link in Resources). There will be two sample tests for you to take. You can do one test on one day, and the other test on the next. It's best to take one, study the parts that gave you trouble, and then take the second test. This way you will be able to note your improvement.
Click on the link that says "Practice Test 1." The test will open in Adobe Reader. It is best if somebody else downloads the test for you so you don't see the answers. The answer guide will be the first few pages so it will be difficult for you to miss it once you open the test.
Print the test out on your computer printer.
Take the test in a quiet room. The entire test will take 2 hours and 10 minutes split up into different sections. You will do critical reading, math and writing on the test. Don't take more than 2 hours and 10 minutes to take the test because that will be the time you have to take the real test.
Have somebody else correct your test for you.