Hi - If you're talking about getting a finished candle flat on the bottom, you could try heating an old pie pan or other pan on low heat and carefully hold your candle at a straight angle. Rub the bottom of your pillar on the heated pan until it is smooth and level. Have a paper towel laying ready to rub and spin the base of your candle on. This will wipe off excess wax and give you a smoother finish.
If you're talking about achieving a flat finished bottom during the pouring process, make sure the surface you are working on is level in all directions. Do your second and/or third pour just to the exact level that you did your first pour until you no longer see a well or a dip develop. This should give you a nice flat base.