How do I add a Damage resistance to a subclass feature?

I have been trying to create a barbarian subclass centred around a raging fire as my first attempt to create a subclass in dnd beyond and I cannot for the life of me figure out how to add a damage resistance as a feature or by adding it to a preexisting feature.

I have seen the post about progressing a subclasses resistance to an immunity but that just says to "add the damage resistance as normal" so it hasn't helped me.

In short can somone more experienced please to tell me how to add a damage resistance to a subclass?

Thank you so much, this was the last thing I needed to know how to do

it seems obvious now but genuinely I couldn’t find how to do it anywhere else and you have saved me from having a guy on fire who would be very susceptible to fire

I appreciate the guidance