2019-08-23 : Bone Miser’s abilities are triggered abilities, not activated abilities. They don’t allow you to discard a card whenever you want; rather, you need some other way of discarding a card.

2019-08-23 : Bone Miser’s second ability isn’t a mana ability. It uses the stack and can be responded to. Notably, this means that you can’t pay a cost of “{2}, Discard a card” by discarding a land card and using the {B}{B} produced to pay for the {2}.

2019-08-23 : If you discard a card as a cost to cast a spell or activate an ability, Bone Miser’s triggered ability resolves before that spell or ability but after you’ve chosen targets for it. If you discard a card while a spell or ability is resolving, the triggered ability resolves after you’ve finished resolving that spell or ability.