That's a hard question to answer as it depends on how many times & what temperature you vape your weed, not to mention the quality of the bud you started with. I make cannabutter and then bake cookies / brownies with it. A good ball park number to start with is 3-4 oz. vaped bud to a pound of butter. That's a large amount of vaped bud to save up, so figure a smaller amount using the same proportions. 1 oz. vaped bud to 4 oz. butter (1 cube) or 1/2 oz. (14g) vaped bud to 2 oz. butter (1/2 cube).
Recipes & techniques for making the butter are all over the cooking sub forum here. Lately I've been using the crock pot method, as it's minimal labor. Whatever method you use will really smell though. Be careful if you live in an apartment.
The stone?? Depends on how much you eat / your tolerance, etc. Eating weed generally kicks my ass if I eat enough. Trying to find the line between a nice buzz & comatose is half the fun of making edibles.
Good luck.