Use wet and dry paper.
Heres what I do when I lap, not saying its correct but it works for me (quick and easy guide
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Tools needed :
400/600/800/1200/1500 paper (usually 2 sheets of each).
sheet of glass or an old mirror
water (an old spray bottle with water is better if possible)
few drops of washing liquid.
Some nurofen/painkillers (I wont need to tell you what these are for)
1. Depending on how bad/good the base is then start with either 400 or 600 paper, the better then finish the higher the grade of paper should be used.
2. Wet the glass that will be your work surface, this prevents the paper from 'curling' the wrong way.
3. Place the paper, rough side up on the glass and asap go to step 4.
4. Put few drops of water in the centre of the paper enough to make a small puddle, then spread the water around. With both sides of the paper 'wet' it shouldn't curl too bad one way or the other.
5. With your paper now 'prepped' add a drop of detergent to the centre of the paper. This acts as an addtional lubricant and makes the actual lapping process 'smoother', you can do it without but you may find that as you move the block towards the edge of the paper it starts to 'jump'.
6. This is probably the most important bit. The aim of the game is to try and get an even amount of pressure to the HS that your lapping so as not to make matters even worse. You must keep it flat and not add too much pressure to one side (creating an uneven lapping job). Don't add too much pressure either as this will speed up the job but can also be more difficult to get and even lap. Simply let the HS do most of the work if its heavy enough, if not then finger pressure is quite enough imo.
7. Here is the hotly debated part! Some prefer to do circular motions others prefer up-down/side-side in rotation. I have found that the former works best in rotations of 10 each - keep switching until the base scractes are more or less uniform i.e no big scratches stand out and the base looks flat. the way to tell is hold it up to the light and if some parts are dull and some shiny then this is uneven, you want a uniform reflection across the base - dull or shiny dosn't really matter. Also make sure that no scratches are deeper than another (fingernail test).
8. Keep going through step 7 with each grade. As each grade of paper is used (you may need 2x each grade as wet and dry will tear eventually!) until you get a uniform reflection across the base of the HS. With each grade the scratches will become finer and finer until with 1200 grade it becomes a dull haze. At this point you should not be able to see or feel any scraches on the base if you have scratches then return to the previous grade.
9. At this point your HS is lapped. Some would argue that going further is pointless and its only for 'bling' purposes. There is a bit of truth in this as the scratches will be so tiny that the thermal compound you will use will fill in those minute crevices but I think the shinier the better as the shinier it is the less scratches there is therfore the less thermal compound you need to fill in those little crevices. I also like bling and shiny copper HS's look cool(even though you never see them when they are installed...call me a perfectionist
10. After you have used you highest grade paper you will probbly not have a mirror finish. Don't worry as some people don't get the mirror finish even with 2000 grit! You now have two choices. You can either use a) a rubbing compound which is similar to using a very fine grade of wet and dry (available from motorspares stores) or a metal polish i.e PEEK/autosol. Imo the rubbing compound is better as it will not leave as much residue on your block whereas a metal polish will act as a weak rubbing compound giving an excellent appearance but it also has additives that fill in the little cracks which is somethng you dont want. If you don't mind taking the time to thoroughly clean your block afterwards then metal polish is ok but for all itenets and purposes and ease of the job at hand, rubbing compound is better.
11. Cleaning your block. You now need to clean the block, clear up your mess first as you don't want any metal fragments scracthing your newly lapped block. Once thats all away heres what I do. Give your block a thorough rinse of as there will no doubt be remnants of the rubbing compound and lapping fragments, even if not visable, on your HS. Once thats done dry off and then clean as normal with the IPA. There really is no substitute to IPA, I have used TIM cleaner/vinegar/nail varnish remover etc and nothing compares to the cleaning ability of IPA.
12. Step back and revel in the reflections your new ultraflat shiny HS gives off
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assuming i can get my hands on some sandpapper at some point think im gonna do that to the base of my AC64, 
