Skim coating a wall is not too difficult. A little time and practice, and the right tools, will make you a pro in no time. The reason a wall is skim coated is usually because of an imperfection in the wall or damage to the wall from the removal of wallpaper for example.
It always a good idea to start by sealing the wall with an oil based primer. This is critical if wallpaper was removed from the wall. The water content in drywall mud will cause old wallpaper glue to come loose from the wall and create a mess. It’s a good habit to always seal the wall to be safe.
The faster method of skim coating is to mix the drywall mud with enough water to make it rollable. Once the mud is mixed you simply roll the mud on the wall and knife or blade it down. A slower method is to use straight drywall mud and knife or trowel it on the wall. The reason you may want to go the slower route is if there is a problem with having additional water content in the mud.
These photos are from a commercial job where we did oil seal the wall and then rolled a coat of mud only to discover before the second coat of mud that there was a second layer of wallpaper that was painted over and the oil primer was not enough to keep some of it from bubbling up. When this happens you simply cut out the bubble areas, reseal and mud again.
On this job we sanded the walls smooth, cut out the bubbles, oil primed the entire wall and the second coat of mud was knifed on to keep the water content down. Just to be safe.
It ended up being perfect once it was top coated with finish.