Get it:

Four patterns of fabric (something not too rough..we're not exfoliating here!) cut into the following sizes: two 4x5", two 4.5x4", two 3x4" and two 1.5x4" rectangles.
Sewing machine--this could be done by hand, but the stitching needs to be super tight to keep all the goodness inside
lavender, flaxseed or rice, essential oils (if you're not a lavender fan)

Do it:

Divvy up your rectangles into two sets. Line 'em up in a pleasing order.

Right sides together, sew pieces with a .25" seam allowance.

Once you have all your friends sewn together into a nice strip, do it again with the other set of pieces. Yay. Front and back. Iron flat.

Pin your strips right sides together. Square it up if necessary. I'm a crooked cutter, myself.

Sew all around the edge, leaving a 1.5" hole for turning and filling.

Trim seams, turn and iron. Poke out your corners with a chopstick.

I like to topstitch my pillows around the outer edge--for two reasons: it looks cuter and more "done" and gives you another seam for strength. 'Cause cleaning up a pool of lavender and flaxseed isn't relaxing. Topstitch all around except for your hole.

Use a small plastic funnel to fill your pillow (I bought a whole set of sizes at the dollar store) or make one from heavyweight paper. I usually mix my filling as 50/50 lavender and flaxseed. If you're using rice, you can add a few drops of essential oil and mix it in. Fill it to desired heft. I like mine to be nice and full but still a bit floppy. Stitch opening closed on machine or by hand.

Ta-da! Put it in the freezer to chill your peeps or microwave to soothe what's aching. Do you like my model? It's rough work being a four year-old.