I made a desk for my daughter's IKEA Lo loft bed. It was her idea to make the desk under the bed, as she likes sleeping high. It also makes good use of the space in the small room.
The desk is height adjustable and if the ladder ever changes to the right side, the desk can be flipped over as well.

Actually most of the work was figuring out the design and the dimensions.

This is no instructable with detailed directions, but should give a good impression of the project and steps required. I didn't plan this to be an instructable, otherwise I'd have taken some more pictures: sorry!

Tools: Circular saw, jigsaw, biscuit joint machine, router w. various bits.

Materials: 2 MDF sheets, some 22mm thick whitewood, 2 sheets of HPL laminate, baltic birch plywood 18mm thick, Rampa nuts, biscuits, glue.

This was a fun project and not too much work, I'm pretty pleased how it turned out.

I suppose the whole project could be done without some of the machines I have (router, Lamello) if a circular saw, jigsaw and a lot of sandpaper is available. The core could be painted instead of glueing on laminate, saving some money as well.

Step 1: Making the Core.

To make the desk rigid but not too heavy I made a cheap hollow core from two 6mm MDF sheets and some whitewood. Making the core was pretty straightforward. I marked the rough dimensions on the MDF and glued on some whitewood 22x48mm. This didn't need to be precise as I cut the core to exact dimensions with the circular saw after it was all glued up. Just make sure the core is a bit oversized to have a nice edge after sawing to final size.
When the bottom was ready I glued on the top MDF sheet: some crates of beer came in handy as weights ;-) The curves I routed with a template and a bearing guided bit.

Step 2: Laminating the Core.

I covered the core with white HPL laminate and trimmed the edges with a router.
I first glued the edge and routed this flush, then I glued and trimmed the top and bottom laminate.
For gluing I used contact cement.
We thought about selecting some pacific blue color, but decided on the boring off-white in the end. It would just have been too much color in the small room...

(Sorry, I have no pictures of the laminating)

Step 3: The Supports.

For the supports and back I used 18mm Baltic birch plywood, same kind of wood as IKEA used for the bed posts. The supports are fastened between the bed posts and support the desk.
The plywood was cut to size with the circular saw. The curves were routed with template and guided router bit.

In the plywood I screwed so-called Rampa nuts to thread the Ikea hex bolts into. As I wanted the desk to be height adjustable I installed some extra Rampa nuts. As the holes in the IKEA bed posts are at 96mm, this means the desk can be adjusted in 48mm intervals.
! Make sure you clamp the plywood from the sides when screwing in the Rampa nuts, otherwise you run a high risk of splitting the plywood!!

In the second picture the right hand support can be seen below the desk. The desk just rests on this support, it is not attached.

Step 4: Space for Climbing Ladder.

On the left side I needed some clearance for feet climbing up the ladder. See also the picture on the intro page.
If the ladder is ever changed to the right side of the bed, the whole desk can be flipped over to right-hand configuration.

Step 5: Attaching the Plywood to the Core.

The plywood was glued to the core with Lamello biscuits, which makes the joint invisible (Lamello biscuits are small wooden small wooden plates which are glued in slots made with a 'biscuit jointer').
http://www.lamello.com/en/products/wood-joining-system.html

Alternatively screws could be used, but the heads would be visible.