Most plein air painters use boards as 
apposed to stretchers. A few reasons for this is that they are more durable and are much easier to frame. I also prefer working on the hard surface. Like a lot of other painters, as my work has transitioned to mostly plein air I have found it better to use just a few sizes. In my case mostly 9 x 12″, this is the size of my Guerrilla Pochade box and It’s an easy size to find frames for. I also use some 11 x 14″ and 12″ square boards.
I’ve always been a do it yourself guy. I learned to build stretchers freshman year at Carnegie and have always built my own. When I started to do plein air my production went way up and I was having to buy ready mades all the time. Good 9″ x 12″boards can cost as much as $12 or even more for linen double primed. I’ve learned how to make boards by 
trial and error and though there are lots of blog entries on the web I found that I had to work out what was best for me, so I will show you what I do.
After trying many different substrates I finally found what I consider perfect for my needs. At Home Depot you can by a plywood product called underlayment. It’s actually Lauan Meranti quater inch plywood and the quality is very good for the price. I buy it in half sheets (46 x24″) for about 5 dollars and have them ripped in half so I come out with 2, 12″ x 46″ pieces. In this case I bought two sheets so I have 4 the same size. Then I 
measured out four nine inch sections and that left me with one 12″ square. I then tape them together and used my skill saw to cut them.— if you have a big chop saw or a table saw you can use that. I use another board as a guide to make my cuts and always follow the rule measure twice, cut once!
So I end up with four 12″ squares and 16, 9×12″— not bad for 10 bucks. This plywood is so good that you could just shellac the good side and use it as is but I like to paint on canvas or linen so I have a few more steps. First of all, always buy primed canvas or linen — I’ve tried priming after mounting and it’s a disaster—so take my advice. I always buy my canvas on sale so I paid about eight dollars for 36 x 63″ piece. This is
so much cheaper than linen, which can be $80 for the same size. I love it but I can’t afford it all the time—if your are just starting out, canvas will be just fine.
Next cut your canvas pieces about 3″ over the final sizes of the boards. Then use Miracle Muck —the only glue that will do! You have to order it from Raphael’s in California, and it takes a week or so (note, 
they can’t ship it when it’s below freezing) but again, take my word for it, this is the only glue that works. One big thing, it’s heat activated—this allows the painting to be removed from the board if needed, that makes it archivable. You will love this stuff if you’re a glue freak like me. I paint the glue on with a bush then roll it out with a roller —not so important but makes it neat.
Then I use an old woodblock roller I have to smooth it out. Take special care on the edges 
because it’s easy to get some separation in a line along the edge—see the photo. I like to wrap my boards, so when I have it down on the front I fold over about an inch and a half on the back—this also gives you some options if you ever need to remove it. Finally I cut out a rounded corner so the canvas folds over neatly. As I show, butter the canvas ends instead of sloping the board up this will keep it clean.
When they are done I put them under a board on the floor with a piece of wax paper 
between each. I keep about twenty pieces of wax paper stored in my painting cabinet for this purpose. I stack them up as neat as possible so the edges align. You need to keep all your like sizes in one pile. Then stick a good size barbell weight on top for about eight hours. Be sure to clean the glue off with water so you’ll be able to use the brushes and roller again. All in all plywood, canvas and glue— each board cost me less than $4.