Here's a similar King size Mission style bed I made shortly after my wife and I were married. I was living overseas on a military base and did not have access to hardwoods of any kind so I made it entirely out of Home Depot type plywood. It took four coats of latex paint to cover all the plys and grain, but it came out great. I got the plans on-line (sorry, I can't remember where). The plans originally called for narrower slats, but I opted for wider slats to make the construction a little easier. I also made the four corner legs 12 inches longer to provide storage space underneath, as military housing does not have a lot of closet/storage space. I've since cut the legs back to normal length.
I used
this bed-hardware from Rockler to attach the side rails; the bed is rock solid as a result. Plus it comes apart, which is important as we've moved three times since I built it.
If you're making a Queen or King size, you'll need a center support to hold up the middle of the bed. I rigged up something that supports the three center slats under the box-spring.
Good luck.
Kevin H.