basically the idea is you split each of the digits in the 1st number up into x number of groups, x being each digit in the 2nd number. then you add the number of groups up as you would when you do long multiplication (e.g. the 1st digit of the 1st number, the 2nd digit of the 1st number + the 1st digit of the 2nd number, then the 2nd digit of the 2nd number). in short, it's the exact same concept as long multiplication, except it takes a lot longer and it's a lot more confusing.