You will need this much space to start construction. Ive used black and white block so its easier to see the space needed.
You will need
5 redstone dust
1 water bucket
1 lava bucket
and some building material/blocks the type does not matter (due to the lava however its best if its not flamable).
Step 1: Step 1
Start by making 2 holes in the wall as shown
Step 2: Step 2
Now add in the blocks as show. I used glass so it would be easier to see what I add.
Also put in the piston block in the wall near the floor make sure it is in the center.
Step 3: Step 3
Build around so that all the inside blocks are covered and no water/lava can spill out the sides
Once again I swapped the blocks out so you can see.
Step 4: Step 4
Add in the blocks so that they make a bridge over the gap in the middle. Keep in mind you will walk on this part.
Step 5: Step 5
You should build a way to get up there the how is unimportant as long as the middle stays clear.
Step 6: Step 6
Now open a hole in the wall and hollow it out so that there is room to work with inside.
Step 7: Step 7
I use this pattern to work with as it is best for easy maintenance and construction.
Step 8: Step 8
Cover the hole and stick a lever in its place.
Step 9: Step 9
Add water and Lava to the 2 holes we made in the beginning and start printing.
Add to it if you want, stairs more ladders, etc etc