From my own experience of painting numerous parts for my car I can tell you preperation is key to a good finish.
The way I paint stuff is as follows. The principals should be the same whether you are painting car parts or a desktop.
First off clean the part with water, then dry the part. Next up is to lightly sand the part to get a key on the surface for the primer to stick to (usually with a scotch pad or a very fine sandpaper). Mask up everything you do not want paint on. Clean the surface prepaint, this removes any oil or residue left on the part.
Spray an even light coat of primer over the part and leave for 10-15 mins (flash time). Then spray your second coat a bit heavier but do not go mad last thing you want is a run in your primer as this will require more work. Leave that to flash as well then spray your third coat.
The best way to spray is to do a 50% overlap to ensure even coatings and keep your can moving.
Leave for 24-48 hours for the primer to cure fully.
Next is to flat down the primer with some 800grit sand paper and then to 1000 grit to get a flat smooth finish. Once done it is time to spray colour.
Same thing as primer really, 3 coats with 10-15mins flash time between coats.
Leave the paint to dry for another 24 hours.
Last is to put the clear on. Basically the same thing as primer and colour
Leave this to dry before fitting the part. You can leave the part like this if you are happy with it but if you have orange peel and want to go that extra step then follow the next steps otherwise your done.
For orange peel you will need 1500 grit and 1800 grit sandpaper. You will need to leave the clear for a few days to before sanding to leave it go hard. Sand the part with 1500 first and use some water. Keep changing the paper so as to stop it glupping up and scratching the surface. Be careful around edges as it is very easy to burn through the paint. Use a sanding block for wet sanding to help keep the surface flat. It is best not to do this with just your hand. The wet sanding block has a softish side and solid side. Use the softer side with the 1500. Next step is to clean off the part with water. Then use the 1800 grit with the harder side of the block. Sand the whole thing and make sure there is no high or low spots. You want the part to be completely even.
Once sanding is done you will need a polishing compound and a microfiber cloth to polish the part back to a shine because the sanding will make the clear dull. What the sanding does is make the ridges more uniform in height and thus this gets rid of the orange peel effect.
It takes time and patience but it can make things look very good once done right.
Here is a few images of some parts I have done
Hope this helps