How to add more elements?

Add what elements and where?

In order to make Relevanssi search post meta fields, all you need to do is to list the names of the appropriate custom fields in the “Custom fields to index” setting on Relevanssi settings page and then rebuild the index.

On the default settings, it has 4 or 5 elements with weight, such as title.
So I want to add more element that I can give weight onto.

Use the relevanssi_match filter hook, you can use that to adjust the weights of whatever you want to.