To get there from I-10, take exit 209 and turn north. Stay on old 51, which is on the ground; not the raised interstate. Go about 5 miles from the Shell station near the interstate exit. Look for a large pipeline on the right and an open spot on the left to launch. (launch is marked 1 on the map) There are many similar spots but this is the only spot on the map where the bayou goes across the road.
Once you launch, you have a few choices. You can do the "Manchac Mansion" route, which is marked number 2. Enter the bayou, take the first left and then turn right into the 2nd finger lake. Go to the back of the lake and there is a small slough. Almost immediately you'll see the remains of Manchac Mansion. If the water is high enough you can take this slough through the forest (prettiest paddle ever) to the marsh. There it opens up quite a bit and the slough continues...I haven't yet been to the end.
Another option is to do the "Trapper's Run" which is marked number 3 on the map. Start the same as number 2, just pass up the two first finger lakes and keep going to the end. It's very very pretty at the end. Go to the back of the lake (right) and there is a slough back here too. On the day we went, the water was too low to take the slough).
A third option is to stay on Shell Bank Bayou and just keep going in the direction of Lake Maurepas (number 4). The bayou starts out pretty wide but gets narrower and prettier as you go. We didn't make it all the way to the lake because of low water, but it was about a 2 hour paddle even so.
No facilities out here, so stop at the Shell Station before you get on the water, or be prepared to do some interesting gymnastics from your boat. Also I would not leave any valuables in your vehicle here; too easy for someone to do a smash and grab.