Did you not get it from the anim? I will tell you here.

Make something, then go to the bottom right corner of the page right diagonal from the colors. There's a plus, minus, trash can, 2 images that look somewhat same, and an eyeball. Press the first icon I show u in anim. The one with the green arrow. Then, add a new layer. While on the layer, press the other one, with the red arrow, then take the moving tool and move the duplicate around. It will be hidden behind the original so you won't think it's there at first. Then just put it in place.

Part 2: Making it a full on transparent without cross the lines. Do part 1 first, then go to your colors. Choose a random color of your choice. The colors wont change anything be aware. Then go to the transparent area, and it make as transparent as you want, the more transparent, the more invisible-ish it gets. Then while your using that color, press the eraser and erase the duplicate 1 time. Now it's transparent as well! This is MUCH easier than it sounds it's only, *+*counting here*+* 7 steps! 1: draw 2: copy 3:add layer 4:paste 5: move 6: transparent coloring edit 7: erase! I hope this worked out for you! If you have questions don't hesitate to ask!