I agree with most of the below site except for Myself I used a Glass Plate with grinding compound to substitute for a Lapping Plate. Later I bought a Lapping Plate but have not had to build any more Injectors.
I used 600 Grit Aluminum Oxide Grinding Compound. There are some who say you should 1200 or higer. The Higher Numbers are finer grinding compounds.
You can use Water or Oil to mix with the grinding compound.
How to Rebuild Injectors
vincewaldon.com - HOW-TO: Rebuild Diesel IDI Injectors