Firstly, build your 'legs'. I just used some cheap 2x4 from the hardware store as well as some offcuts I had lying around. The nail plates are probably optional in hindsight, as you can just skew nail the timber together and the join will be just as strong. I only used the plates on one side here and skew nailed the rear.
Things to watch out for:
1) Make sure the gap at the top of each 'leg' is big enough to accommodate your wall fixing (in my case it was a length of 2x6 timber upon which I hung the bench). The area I mean is shown by the green arrows.
2) Make sure the legs are less than half the width of the bench you have so that they will fold in without obstruction. It's important they're about 4" shorter than half the width of the bench so they've got space to fold out and still be under the bench (not beside it).
3) Also make sure that the width of each 'leg' is of course not more then the depth of your bench or they will stick out the front.