This is a nice and easy exercise if you have just started using a overlock sewing machine.
This tutorial is for a 16in square cushion using a Serger sewing machine ( and an invisible zip).
Step 1: Tools and Materials
Fabric Scissors, measuring tape, pins, pen or tailors chalk, thread (colour will depend on the colour of the fabric you choose), 16in invisible zip and half a yard of fabric (Pattern/colour of your choice) and a 16in pillow insert - most fabric shops will stock all those materials and in case you're in the SF area, I recommend http://califabrics.com/ on 2109 Mission St.
Please note that, if you're planning on making this cushion in any of the TechShop locations, you will need to have passed TechShop's Industrial Serger Sewing Machine SBU.
Step 2: Cutting your Fabric/Pattern
Please note that my choice to cut the fabric the same size as the pillow insert is so, the finished cushion has a 'fuller' appearance.
Step 3: Stitching the Zip
Run it through the sewing machine, being careful to remove the pins as I you go along - as to make sure you don't break the machine, nor the pins (if you are not confident you can control the speed of the sewing machine you can use masking tape to keep the zip in place rather than pins).
Once you have done one side, do the same for the other square of the fabric.
This will be the hardest part of the whole job so pay attention and don't rush.