Go to Parts > Part Master and search for the part number you want to make an equivalent for, then click on its part number:
Click on More Info:
In the Settings menu on the far right-hand side, click on Equivalents:
Type in the part number that you want to add as an equivalent, then click on add to add it. Predictive search should show the full part number. If you don't know it, click on the magnifying glass to search for parts using wider criteria:
You have now added the part as a suggested equivalent. This will allow you to see equivalent parts when making a Counter Sale and choose the equivalent in the place of the main part number. This is the extent of the equivalent functionality at this time. Note that you will only see the following highlighted button before you click Add Part on the Counter Sale.