When I do chairs seats I do similar to Jims drawing, but dont forget to check your table top is level across and along (if thats important to you that is

) I prefer to scribe (draw) the cutting line with a block of 4x2 (on its side) as its more stable than a ruler on edge. I use a flat sided carpenters pencil and place the block alongside then scribe all round each leg. I rest on a manufactured 3 foot by 2 foot, 2 inch thick concrete paving slab, well flat enough for my purposes. If there is a bit of a wobble I use various small wedges to get it immovable and level (I sight the mortar lines on the adjacent walls, dont bother with a bubble ruler)

If your furniture is bigger use a cast iron table top on a saw, or set up 2 level steel temporary "I" joists with wedges or something-be inventive

When I trim off (with japanese shark saw) I always chamfer the cut edges with a chisel, it prevents splitting off of end grain if the chair is slid across the ground in use. Theres some pictures of it in my gallery, cant get them into the thread as yet though:D
Hope this helps
