find your sewer pipe, get a sthil saw with a stone blad on it, or even a 9"angle grinder, carefully cut a square hole in the crown of the sewer pipe ( or if you wanted to be real professional, build a small inconspicuous manhole type thing) as the last bit of your pipe, use an offset bent, its not a right angle so wont creat a hard corner for your waste to travel down, but will gently direct it with the flow of the existing pipe, marry it up to the sewer pipe, make a mix of concrete, and seal off the join of the two pipes carefully ensuring that little or no muck goes into the sewer pipe, sou