A septic tank construction project is for you if you’re looking for a challenging do-it-yourself project that could have costly and smelly consequences if you do it wrong. A large part of building a septic tank is planning. No matter how well-built the tank, it’ll be useless if you put it in the wrong place or otherwise fail to account for certain factors.
Familiarize yourself with local laws regarding septic systems. You'll probably need to have an inspector look at your plan and your work.
Determine where to place the tank. You need to be able to dig a sufficiently large and deep hole to contain the tank, and you must also make sure that the tank won't end up sitting under water at any point during the year.
Connect the septic tank to your plumbing system and hope for the best. If you did everything right, you can count on a reliable system. If you did something wrong, you'll know when your drains or your yard start to stink.