Determine the location and size of the arch you're going to build. Make it an integral part of a newly built house, or build a retrofitted arch into an existing house in order to make two rooms out of one.
Snap a chalk line along the floor where you want your arch to go. By lining the cans up in an accurately straight line, you will make your arch stronger and less likely to tip over.
Move everything away from the area before you begin building. Take anything off the walls where the ends of the arch wall will be meeting them.
Build a sturdy wooden form that is the size and shape that you want your arch opening to be. Place this form where the arch will go and reinforce it by propping 2 by 4s against it and screwing them into it.
Lay the first row of cans along the floor with their ends against the chalk line. If you want the arch wall to be as strong as possible, mortar all of the cans together. At any rate, you will need to mortar the cans together around the arch in order to keep it supported.
Continue stacking the cans row by row, with the ends of the wall supported by the adjoining walls, and the sides of the arch supported by the form.
Stack the cans across the top of the arch form when you reach the top, being sure to securely mortar all of the cans that go across the top.
Stack the cans all the way up to the ceiling.
Remove the arch form carefully after you have allowed the mortar to dry for 24 hours. Enjoy your arch!