The idea is quite simple: cut four bits of recycled wood with mitred corners and join then with corner brackets that are also used to secure the painting.
The main thing to keep in mind is that it is supposed to be a bit rustic, so don't worry if it's not perfect.
Tools and materials used:
Phillips head screwdriver
Assorted countersunk and flat screws
Recycled or scrap wood
Step 1: Measure and Cut
Sounds simple enough, but you need to add a small gap (or a bigger gap for a more pronounced floating effect). I added an extra 5mm (2.5mm for each side) which allows you a bit wiggle room, but there will probably still be a bit of pushing and squeezing to get the canvas in. So now it measures 920mm x 725mm.
So that's not too hard...but, now we need to add the distance between the edge of the circular saw and the blade. The best way to do this is to angle the saw base to 45° and cut a bit of wood and measure from the guide to the cut. For this thickness of wood and my saw this meant adding an extra 88mm. So now it measures 1008mm x 813mm.
***a note on cutting - it makes it easy to cut straight across the wood by clamping a quick-square to the wood and using it to guide the saw****
As you cut, try the joins together to see if it look square, but don't worry if it's not perfect - remember that it's supposed to be rustic!
Step 2: Stick It Together
Use an off cut to help line up the bracket on one side at a time (i.e. work on the top and bottom OR the sides) and mark it out (use a scribe as a punch if you want).
Pre-drill and screw the brackets in place, paying attention to the orientation of the flat side of the bracket so that it faces the front (so that the canvas can sit on it).
***handy hint: use a bit of tape on the drill bit as a depth gauge***
Step 3: Put the Sides on and Pop the Canvas In
Pre-drill as before and then screw it all together.
***note on screws: using counter-sunk screws will centre the bracket hole and hold it in place really well...if you screw it up (no pun intended), you can get a bit of wiggle-room by using a flat top screw, which doesn't hold as well but frames don't really get a lot of mechanical stress***
Have a look over it to make sure you are happy with the way it fits together. You might need to sand the corners or the front of the mitre joins to improve the alignment, but don't stress too much - it's supposed to be a bit rough.
Pop the canvas in and line it up (pack some spacers/off cuts around the edge to help line it up) and then pre-drill and screw from behind.
Step 4: Finished!
If you want to finish it off a bit nicer you can oil or varnish the wood, but I'm quite happy with it having an unfinished rustic look.