Bundle products are created in the same way as normal products, however you must tag the product with all the contained items in the following format:

meta-contains-product-handle

If you want to specify a particular variant, you must add the variant ID to the end of the tag as follows:

meta-contains-product-handle-variant-00000000

If you don’t specify a variant and the product you include contains variants, the first variant will be chosen by default.