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Input your first number. Start adding from the left. Complete an exchange. Count your beads to get the answer. Subtract by doing the addition process in reverse.
Say you've got to add 1234 and 5678. Enter 1234 on the abacus by pushing up four beads in the ones place, three in the tens place, two in the hundreds place, and one in the thousands place. The first numbers you'll add are the 1 and the 5 from the thousands place, in this case moving the single bead from the top row of that column down to add the 5, and leaving the lower bead up for a total of 6. Likewise, to add 6 in the hundreds place, move the top bead in the hundreds place down and one bead from the bottom row up to get a total of 8. Since adding the two numbers in the tens place will result in 10, you'll carry over a 1 to the hundred place, making it a 9 in that column. Next, put all the beads down in the tens place, leaving it zero. In the ones column, you'll do essentially the same thing. Eight plus 4 equals 12, so you'll carry the one over to the tens place, making it 1. This leaves you with 2 in the ones place. You're left with a 6 in the thousands column, a 9 in the hundreds, a 1 in the tens, and a 2 in the ones: 1,234 + 5,678 = 6,912. Borrow digits from the previous column instead of carrying them over. Say you're subtracting 867 from 932. After entering 932 into the abacus, start subtracting column-by-column starting on your left.  Eight from nine is one, so you'll leave a single bead up in the hundreds place. In the tens place, you can't subtract 6 from 3, so you'll borrow the 1 in the hundreds place (leaving it zero) and subtract 6 from 13, making it 7 in the tens place (the upper bead up and two lower beads). Do the same thing in the ones place, "borrowing" a bead from the tens place (making it 6) to subtract 7 from 12 instead of 2. There should be a 5 in the ones column: 932 - 867 = 65.