But, let's get on to the saw guide.
Step 1: Base and Guide
For example's sake, let's say the board that you straightlined is about 4 inches wide, and your saw's base is about 8 inches wide. The board you choose for your base needs to be the width of your guide board, plus the width of your circular saw footplate x 2. [Guide board width + (saw base width x 2), so in this example 4" + (8"x2)= 20" wide]. The length of these two boards depends on how wide a board you want to be able to cut.
Ok. Mount the guide board to the base, centering it as close as you can. this does not have to be uber-precise, but does require that on one side when you put your saw on the base up against the guide it will cut the entire edge off.
The cutting of the edge is very important- this is how this guide is so much more precise than other guides that require you to measure the offset and yada yada yada to get the guide to put the saw where it will hopefully cut..
The first picture shows the assembly uncut. With the saw that you plan to use the guide for snugly up against the guide, cut the excess wood off of the base keeping the saw along the guide. The second picture shows the saw part of the way through the cut.