The best place to begin sectioning is the front of the hair line. These sections will be the most visible so making sure they are even and seeing that they work out to have enough room is important. One of the first decisions you need to make is where to put your part. It can be smack in the middle of your head or you can have a row of dreads down the middle. Don't spend too much time on this one because in the end both ways work fine. It's more a matter of personal preference than anything else. One guideline is to follow any strong natural parts you may have. I have yet to hear of anyone that did a row of dreads down the center and regretted it.
your hair is an important step to making attractive dreads. It's normal for sectioning to take a while, especially if you're making smaller sections. Getting the sections right will pay off later so relax and take your time. There are two advantages to clean, even sections. They create round dreads of equal size which will lock up at the same speed and look great. Even sections will also make finding and eliminating loose hair much easier.
Your goal is to make clean sections that are as close to square as possible. This sounds easy but tedious would be a better word. First you're working on a rounded surface that's probably moving, hairlines are rarely a straight line, so close approximations and occasional compromises are sometimes necessary. Not to worry though, as long has they are the exception and not the rule the dreads will come out great.
The easiest way to section is probably row by row.
You'll start in the font at the hair line. Measure the width of one section back from the hairline towards the back of the head. This example will use sections that are 1" x 1" square. So you'll measure 1" back from the hair line. Real difficult right? Now using the dread comb pull all the hair forward of this line towards the front and all the hair behind this line to the back. Use clips to hold the hair back out of the way. You should now be left with one very long section that stretches across the front hairline and is one inch wide or whatever dimension you chose for your sections. This first row can now be divided into 1" squares. Since it's unlikely that your hairline will be exactly 7" and be easily divided in to seven perfectly square dreads you'll most likely have to adjust them a bit to make sure they all come out about even. Depending on the shape of the particular hairline it may be a good idea to have the sections on the side of the head in another row in front of the first row.
After you create each section use a rubber band to hold it in place. They won't need to be tight, just snug enough that they won't slip off while you are sectioning the rest of they hair. Tight clips can also be used. When you are ready to back comb they will be removed. After the first row is sectioned off remove the clips and create the next row. When you've worked your way to the back of the head you are ready to backcomb
Sections can also be offset so each row is 1/2 a section offset from the adjacent rows. This makes a pattern that looks like bricks. This is sometimes done to make the section lines less visible. Since section lines fade away as the dreads mature this isn't really necessary but it's definitely an option to consider if you are worried about section lines being visible. It takes a little longer to section with the brick pattern but the difference is small. One advantage to the brick pattern is that instead of having four dreads meet at their corners you won't have more than 3. This may make it a little easier to decide where loose hair should go but I'm not sure that this is going to make a noticeable difference. I think that brick sections are more of a style choice than anything else. Either way works great. =]