To create your Modifiers or Add-ons, you must first go to the Square Item Library, here's a direct link to it: https://squareup.com/dashboard/items/library.
Once you're there, go to the Modifiers section as shown in the screenshot below.
Next, click on the Create a modifier blue button.
You are now ready to create your Modifier. The following screenshot is part of what you will see once you are creating the modifier. The option boxed in red, is where you will name the modifier.
The highlighted parts show where you will add the options of the modifier, e.g. Egg, Avocado, Pickles, etc. (you can add as many as you want).
You will also be able to add a price to each option in case you want to charge extra if the customers select this.
The screenshot below shows the other options you will see available when creating the modifiers.
If the option Customer can only select one modifier is disabled, customer will be able to add as many options as they want to their order. If it is enabled, customer will only be able to add one option.
You will also need to select the location(s) you want this modifier to be available at.
Once you're done, click Save at the top right corner. You are now ready to apply the new modifier to the items. To do this, click on the Apply to Items option boxed in red.
You will see a pop-up screen showing all the items you have available in your Square Item Library, you will just need to select which item(s) you want that modifier to be applied to and then click on the Apply to Items option boxed in purple.
All you need to do now is reimport your menu to see the new modifier added to your selected items!