If you have the budget for it, you should buy better quality fitted bedroom wardrobes and get them installed professionally. But if your finances are tight, you can still save a lot by buying and installing cheap fitted bedroom wardrobes yourself. We will not go into detailed procedures here but will look at some generalities.
First, make sure you have the tools you need. Best to borrow or rent some kind of multi-tool. Something like the Fein or Bosch power tool is good – you can cut wood with it, you can drill holes and you can also do polishing, sanding, and scraping. Make sure you buy spare attachments for this purpose – if you are not familiar with the tool, you will wear them out faster than expected, and you may even break them. Get your memory foam mattress ready is better. You will probably need good quality glue – at least wood-on-wood and wood-on-whatever your wall are made of. You will want some clamps to use with the glue, and you need a decent work-bench, both for the gluing and the wood-cutting. You’ll also want some kind of putty to fill in holes in your walls and floors – make sure you buy the right type for the material of the surface.
Once you have your tools ready, you need to prepare the wall, floor, and ceiling where you intend to have your bedroom wardrobes fitted. Use the power tool with the scraping, grinding or sanding attachment to get rid of any organic stuff like wallpaper or carpets. If you leave that in place, they will grow mold and other nasty stuff in the darkness after you install your wardrobe. Once you have your walls, ceiling, and floor bare, fill in any gaps and holes with the putty. Leave the stuff to dry while you take your measurements.
Cut the scribing panels to size, using the power tool with the wood-cutting attachment, based on your measurements. Cut them a little larger, and use the polishing attachment to reduce them to the right size during the final fitting. Once you are done here, you can put the carcass, i.e. the wardrobe interior together. Follow the diagrams in the instruction sheet, and place all the parts on the floor in the proper relation with each other. Only start the assembly when you have everything laid out clearly, otherwise, you may end up with some panels facing the wrong way when you do the assembly. You can then screw them together and place the whole carcass against the wall. Because you bought a cheap wardrobe, you may need to re-enforce some of the joints with glue before you screw them together. That’s another reason you want the clamp and work-bench. Do remember to let all the glued parts dry properly.
Depending on the design of the fitted wardrobe you bought, you may need to drill holes in your wall to screw or bolt the carcass in securely. Make sure the hole you drill is slightly smaller than the diameter of the screw. From embarrassing experience when I was learning carpentry in school, it is a common mistake to use a drill bit that is too large. Once your carcass is fitted securely, you can put in the scribing panels. Once again, how you do that depends on the design of the wardrobe you bought. But one common fixture is to use an L-shaped piece of metal with holes in it. You then screw one end into the scribing panel, and the other end into the wall, ceiling and/or floor. Finally, fit the door in, which may entail additional drilling and screwing if you have a hinged door.
This is just a summary. If you have “played” handyman before, there is nothing new to you – mostly common sense. But if it all seems overwhelming, maybe you ought to pay to get your installation done professionally.