Legs for the cabinet (or you could wall mount) I used 15 cm legs so that the sliding shelf is at 76 cm high for me.
Two pieces of wood to keep the top lid from falling down when closed (I just used those wooden sticks that I got for stirring paint)
2 sliding bolts (to help to keep the lid open) and 2 small blocks of wood
I also used spacers (not sure if this is the correct term) to help close the gap of the doors. I saw the IVAR Gap kit online, but this also works.
Plunge saw to shorten some of the shelves, cut out the part for the feet (you could also use a jigsaw )
Electrical screwdriver with different bits to drill holes and tighten the screws
IVAR cabinet with a hidden desk
Decide how high you want your desk to be. Take into account that you need space to open the lid with the monitor and that your desk height will be the height of the sliding shelf.
You need to drill similar holes following those that are in the side panels, but a bit lower (I did at 12 cm lower). So you can later attach the top part lower.
Cut the cardboard panel so it will fit the new smaller size.
Put together your cabinet following the IKEA instructions, but with the top part into the newly drilled holes.
For the new lid, you need to slightly shorten one of the shelves. Just a few millimetres, so you can open and close without friction. You also need to drill holes along the side panels to match the holes at the furthest back position in the shelf.
Attach the pieces of wood to the side, so the new lid can rest on it when closed.
Attach your new lid with the 2 extra IKEA bolts. You should now be able to open and close the lid without friction.
Attach your monitor, make sure the lid can still open and close.
You can cut out a part of the bottom panel, so you have some extra space for your legs.
To help keep the lid open, you can screw the two block of wood to the side where the lid opens. Drill a hole in both of them for the sliding bolts and attach the sliding bolts to the lid on the inside. Also make sure that you are still able to open and close without obstruction.
For the sliding shelf, it all depends on the sliders you buy. I used sliders that are attached to the shelf above. And made sure it could still slide without touching the IVAR doors. I needed to take off some of the length and width of the shelf.
Paint everything as you like, including the doors. (Easier to paint the inside of the desk cabinet before you attach the sliding shelf, is my experience.)
Attach the shelf to the sliders and attach your sliding shelf to the cabinet. Make sure to have enough space to store your keyboard and mouse.