Through Bundle Builder, you can select both optional and required products to add to your bundles:
Go to Bundles.
Select the bundle that you would like to edit.
To add products to your bundle, simply select your chosen products, and then click Add.
Under Required products, you will have the option to add products which are part of the bundle by default and cannot be removed from the customer's basket - you can do this by selecting Add required products.
If you would like the base price to be displayed in your bundle summary, select Show base price.
If you go into Admin > Products and select Delete product for a product which is being used as a required product for one of your bundles, your bundle product will be kept in place but will no longer work properly. In this case, you will receive an email notifying you.
You can choose to add variants of your selected products to your bundle also, by heading to Bundles > Products, and then add the variants under Products > Add products.
These variants will then be listed as separate products on the bundle:
You can choose to add variants of your selected products - you can also add the required quantity of a product (or product variant) by heading to Add a variant.
The actual process of adding products will vary slightly according to which bundle structure you use: