Buy all of your thread at once. Even if you're working with white cotton crochet thread, there may be differences in texture and color that will affect the quality of your finished tablecloth.
If you're working a tablecloth in one piece, invest in a row counter to help you keep your place in your pattern.
Work a test square in the main pattern to check your gauge. Gauge is the number of stitches and rows that you get to the inch of work. Because every crocheter works differently, your gauge may vary from the gauge required for the pattern. Crochet a 4-inch square in the main pattern stitch, and count the number of stitches to the inch. If it's different than the stated gauge for the pattern, repeat with a smaller or larger crochet hook until you get the right gauge.