This is a really easy build which only requires simple tools (however power tools will quicken things), the bed is made out of whitewood pine so it can be painted, stained or left untreated.

Parts

1x 9mm plyboard ( 1200 x 610mm | 4ft x 2ft )
3x 18mm pine whitewood ( 140 x 1800 mm | 5.5" x 6ft )
3x 18mm pine whitewood ( 180 x 1800 mm | 7.0" x 6ft )
M5 100mm Wood Screws

Tools

Mitresaw or mitre block
Set Square
Tape Measure
Pencil
Drill
Screwdrivers
5mm Drill Bit
CounterSink
Assorted Sandpaper

Step 1: Step 1: the Frame

To make the sides begin by measuring out 1400mm cuts on two pieces of the 180 board, you can use the piece of plyboard's length to do this, cut the boards and put the offcuts to one side (these will form the headboard).

For the top and bottom you will need to measure out two equal cuts of ~646mm on the remaining board, or if lazy just mark a line at using the width of the ply and and 36mm (18mm x 2).

Drill and countersink four holes into each 646mm board at either ends. Lay your piece of ply down and butt up one side and top, using the board to keep things straight. Screw together!

Next repeat the same step with the bottom board, and then again with the final side to form the frame of the bed.

Step 2: Step 2: the Struts

Measure out and make 4 cuts of 610mm on the 140mm board to make the struts. Two of the struts are screwed directly to the top and bottom boards of the frame respectfully, the other two are placed equally inside of the frame.

Roughly layout the boards in the frame and drill, countersink and finally screw them onto the frame, for the end pieces it may be easier to lay the frame down on its ends.

Step 3: Step 3: the Feet

With the offcuts from step 2, cut 4 equal lengths to the desired height. Attach them to the frame sides but remember that they should not be placed any higher than the struts (otherwise the board will not lay flat in the frame!)

You can now lie the plyboard sheet inside the frame, if everything went right it should slot it with minimal mechanical retention.

Step 4: Step 4: Headboard

The headboard is constructed using the offcuts from step 1 and 2, and will not need any further cuts made to them. Lay the two boards on top of each other horizontally and then lay the further two offcuts at either end vertically so there is at least a 40mm overlap at the end. Screw all the boards together and then attach to one end of the frame.

Step 5: Step 5: Finishing

You will want to sand every surface and insure that all sharp edges and splinters are removed. Whitewood is quite soft, so running over it with 60git and 120git will suffice.

Finishing is really up to personal taste, I chose 4 levels of sanding and a gloss varnish.