The "Munson Plan" is a good guide, but I think people can become too obsessed with the weight of their snake and the weight of the prey.

The easiest thing is to feed it a prey size that leaves a little lump (something that is ~1.5x's the widest part of your snake's body at the widest point). Keep feeding it that size until you can no longer see a lump. Then move up to the next prey size and do the same - feed until the lump is gone and move up.

Continue this until you get to large/jumbo mice. Once you get the lg/jumbo mice and you don't see a lump (if they get that big) I stop there anyways, as at that size a large mouse is still plenty.

This way is easy and doesn't require you to worry so much about weights.