I only drain tar pits that have a nearby stream or a near a coastline. I prefer to dig backwards from the water, which is going to eventually be the lowest point of the ditch. For some reason digging backwards is a more effective way to get a slight slope without bumps. Dig towards the tarpit, try to chose the straightest route that has no large rocks with their tops just peeping above the ground. Make a gradual inclining ditch towards the tarpit.

WARNING: Make sure to begin this project immediately after killing all the growths residing in the target tarpit.

Once you reach the tarpit remove the final wall of ground and wait a few minutes for it to drain. There will probably be a small pool left in the tarpit, in which case remove some more material where the tarpit meets the ditch. It may be necessary to do some more work on the ditch to remove high spots or increase its overall depth.

Most tarpits have a low point, with a depression in the ground that runs to a water source. This is the most common sense route to dig one's ditch, so look for the slight ravine that runs from the tarpit to the coast or the creek.

Draining a tarpit will not eliminate the 2-3 growths that live there, so be cautious when approaching tarpits you have drained as the growths will respawn every 2-3 game days.

Prior to writing this I drained 2 tarpits and got 173 tar. It is best to take a look at the tarpit with your eyes level to the surface. Look to see if it has big balls of tar at the surface and/or just below. Tarpits with lots of these big globs will provide the most tar. The ones with the big globs can provide around 100 tar, ones without 18-30.