You could also use mortice and tenon joins but these require a lot more complex maths than I usually employ when building things. A simple pilot hole and screw is my usual modus operandi and it’s worked out pretty well so far!
If you don’t have a pocket hole jig then a pilot hole with a larger hole drilled in to get the screw to the right depth is also good. To do this, first drill a hole roughly halfway through the timber and then drill a smaller pilot hole to accommodate the screw. This way of joining timber is good for when you don’t have the appropriate tools.
Another way to join timber is by using a piece of dowel. Clamp the pieces together as required, drill a hole big enough for the size of dowel you are using through both pieces, clear out the sawdust, apply some glue to the hole and pound a piece of dowel in. Cut out the excess and sand smooth and you have a good, solid joint.
To start with, take one of the back legs, one of the front legs and two of the side rails. Mark a line 16” up from the bottom of the back leg and transfer this mark to the front leg by lining them up together and drawing across using a square.