Familiarize yourself with the answer grid. Most of the answer grid offers the options A, B, C, D and E. Look over a few grids before the test by buying a GED prep book. Taking a few practice tests will help you get used to filling in the answer grid. A video linked in the resources section demonstrates how to fill in the grids.
Darken the circle completely, staying within the lines. Check marks and X marks will not be read as correct answers. Avoid stray marks because they may be counted as selected answers. Only fill in one oval per answer.
Make sure you are answering the right question. It can be easy to skip a question and forget to skip the corresponding grid space. Since you are not allowed to write in the GED booklet itself, it may be a good idea to quickly jot the numbers of the questions you are skipping on the scratch paper, somewhere you can easily find it.
Practice the alternate grids of the math section, where you must supply your own answer -- the standard grid and the coordinate plan grid. On the standard grid, write your answer in the five blanks at the top. Fill in the corresponding answer bubble below. Leave the bubbles blank if you do not have anything to put in them. Use one whole space for a decimal point or slash, the mark in fractions that looks like this: /. Negative numbers and mixed numbers can never be entered in this type of grid. A mixed number such as 9 ¾ would be written as either 9.75 or 39/4. On the coordinate plane grid, you will have spaces corresponding to points on a standard coordinate plane -- points plotted on an x and y axis. You must only mark one answer.
Remember that for math questions, it does not matter if you start filling in the numbers from the left or the right. There are five slots, and you could fill in a three-digit number such as 313 by filling in the first three slots and leaving the last two blank. You also could fill in the middle three slots and leave the first and the last blank, or fill in the last three slots and leave the first two blank. Never spread out the numbers; they must be grouped together.