Attach the new magnet (in this case 2x 2mm*4mm disc magnets, original was a 4*4mm) to one of your useless bits. I used the triangle one

You may want to add a dab of glue to the back of your magnets. I didn't feel the need, a press fit was all that was needed

Insert the bit. It may not go all the way to the required depth

Place the screwdriver, cap side down, on your makeshift anvil/surface

Give it a good few hits with the hammer, to push it back in.

Hits sound different when you're all the way in. When that's the case, you can't go further