First, fasten the slate to the table, usually done with lag screws.
Second, make the surface flat. Stretch two strings across the table lengthwise and staple it into the frame on each end (see illustration below).
This string is going to be a guide to help make the surface flat. You cannot have a level table unless the surface is first flat. Put a quarter underneath the string on each end. This will allow some space between the string and the slate so that it can be used as a gage. Use a fifth quarter as your moving gage. Your goal will be to have the same distance between the string and table for the entire length of the table. The most critical areas are the two breaks in the slate. Any type of ledge at these two points will create a noticeable seam in your cloth.
Use your fifth quarter as a gauge and shim accordingly. You can also use your fingers to feel for any edges along the seams. If the slate is high, torque it down with the slate screws or shim it up with your wood shims. Hardwood shims are useful for this job because they do not compress easily. You can also use playing cards to shim your slate. They do not compress at all and they work very well for those slight adjustments.
Third, level the table. Now that your surface is flat, you can actually level the table. Take a any level and check it lengthwise first, then across the width. Use a small pry bar to lift each leg for shimming. Wood door shims work great for leveling a table. They are about 16" long and they chop off easily with a chisel. This part is fairly simple if your surface is flat.
Fourth, seal seams with wax. The table is now level and in its permanentnant position. Wax does not adhere to cold slate very well. Use a propane hand torch to pre-heat the seam, then drip wax onto the seam and spread it evenly as shown in the picture to the right. I usually spread the wax 1 to 2 inches on each side of the seam line. Let the wax cool for about 30 seconds then scrape the wax off with a paint scraper. Be sure not to let the wax cool too long. It will harden quickly, which makes it very difficult to remove. Once your wax is removed, use your fingers to feel the seam for any inconsistencies. The basic function of the wax is to make the seam disappear as if it were never there. You should not be able to feel any edge, dip or bump on the seam. Also, remove as much wax as possible from the table. Excess wax will work its way into the bottom of the cloth, which may later create a line or discoloration. You are now ready to put the cloth on your table. Clean the surface with a damp rag then use your bare hands to sweep the entire surface. Your hands will pick up any inconsistencies on the slate. This small step is critical, there is nothing worse than finishing your table and then discovering a small splinter of wax or wood under your cloth.