To begin, we'll start off with which guide is the top guide and which is the bottom guide. The top guide is the guide on the spring end of the balance. The bottom guide is on the end with the pulley and clip.
Block and Tackle balance top guide, note that it is on the end with the spring
Block and Tackle Balance bottom guide, note that it is on the end with the pully and clip
To select your top guide, you simply need to determine if it has "wings" or not. The wings are small tabs on the side that help keep the balance more secure. These are the only two options for the top guide.
Winged Top Guide (left) and Non-Winged Top Guide (right)
To select the proper bottom guide, first, you will need to determine if it is winged or non-winged, just like the top guide. There are several different sizes of bottom guides so you will need to take one measurement, the depth of the guide. Generally speaking, there is one guide that is far more common than the other sizes. These are marked "most common" on the product page, but still, measure your guide to make sure it is the correct one.
Winged Top Guide (left) and Non-Winged Bottom Guide (right)
Measuring the depth of the bottom guide. This guide measures just under 1-1/8" and is non-winged indicating that it is the most common non-winged bottom guide.