First print out or draw a template for your stamp.
Take care it's not too filegree.

You need some carving tools.
I got mine for about 2€.
Most important are a v- carving tool and one or two half-round shaped ones.

[edit]
Even better are linocut tools.
Those are pretty sharp and you get to carve more detailled.

Cut out the template.

Place the template on the eraser, fix it and draw around it.

Now you see what you have to carve out.
(We're gonna make a positive stamp here, so we carve the black part. If you want a negative stamp just place the template on the eraser centred and then carve out the white part.)

Cut the eraser into the right size.

Start carving carefully with the v- carving tool on the outlines of the template.

Go on carving, still be careful...

When you are done with the outlines carve out the rest.

Let's test the stamp to see if it needs more accurate carving.

Well, yes, it still needs more attention.

Test and go on carving until it's like you want to have it.
Take care there's nothing under the paper where you test the stamp, otherwise it looks as busted as my #2.

The End :)