You can buy page protectors (a.k.a. sheet protectors) that are already divided into sections (a.k.a. pockets), but they tend to be much more expensive, and the shapes/sizes of the sections may not suit your needs. Here are two easy ways to customize your page protectors yourself.

You will need:

Page protectors
Thick cardstock or thin cardboard
X-acto knife
Ruler
Sharpie
Woodburning pen

For a no-heat alternative method, see step 4.

Step 1: Draw Your Sections

Take a piece of thick cardstock or thin cardboard and cut it to fit snugly inside your page protector.

Using a ruler and a Sharpie, draw lines on your cardboard to indicate section boundaries. You can make as many sections as you want, but I recommend between 2 and 6 sections total.

For my project, I divided the cardboard in two, then divided the top section into two again.

Step 2: Cut the Page Protector

Insert the cardboard into your page protector.

For all of the horizontal lines, lay your ruler on the page protector and carefully cut along the horizontal line with your X-acto knife. Try not to cut the cardboard, as you will want to reuse it for more pages. Don't cut the white section on the left that contains the binder-ring holes.

For all of the vertical lines, trace the lines with a Sharpie (you can remove the Sharpie later with rubbing alcohol) or some other pen.

Step 3:

Plug in your woodburning pen with a pointed tip.

Remove the cardboard from inside the page protector and place underneath the page protector.

For the horizontal lines, use the woodburning pen to make a dotted line above the cut in the page protector.

For the vertical lines, use the woodburning pen to make a dotted line on top of the line you drew with a Sharpie.

Tips:

Make the dots fairly quickly; you don't want the holes to get too big.
Use a low heat setting if possible. If you can't change the heat settings, let the pen heat up, then turn it off and use it. When it stops melting the page protector, turn it on again.
Don't make your dots too close together.
While you're working, reach your hand in and test to make sure the two sides are sticking together. If they're not, you may need to use more heat or more pressure.

Step 4: No-Heat Alternative Method

You will need:

Page protectors
Thick cardstock or thin cardboar
X-acto knife
Ruler
Clear tape

Follow steps 1 and 2, but use the X-acto to cut both the horizontal and the vertical lines.

Taping your sections can be tricky if you have vertical cuts. Start with the sections that are on the left of the page and tape all of their edges down onto the bottom sheet of the page protector. Work your way right, taping each section on top of the previously taped sections.

For horizontal cuts, tape the sections above the cuts to the bottom sheet of the page protector.

Step 5: Fill Your Page Protectors

What do you need all these sections for? Here are some ideas:

Organize your stickers or stamps
Organize your greeting cards or postcards
Organize your coupons
Organize your recipes
Hold your photographs
Hold your collectable cards
Fold up papers, articles, or documents to save room in a binder and keep related documents together