Step #1 get your things setup.

Step #2 place piston L

Step #3 set piston group to MERGE (at bottom)

Step #4 setup piston length

200 works for a 1 block gap but you can put any length in.

It will look like the picture to the left until the next step is complete
Step #5 set transparency

(after step #5)
Step #6 Profit