Sure, no problem!
Generally I recommend looking at the hooks section of the knowledge base, most hooks are documented there.
Relationship fields should be indexed automatically when using the index table engine. It is not a 100% support unfortunately, so you will have to test, as every relationship field is a bit different. Generally speaking the plugin will try to fetch the values.
If you want to do some manual stuff to add to the index, then I would recommend using this hook. This one triggers whenever a post content is being indexed, right before it’s passed to the tokenizer. You can use this hook to change/add text to the content field. Generally it is a good execution point to add contents from custom fields or whatever field you wish, then the plugin will take care of the rest.