Start by cutting the jagged edges of drywall paper and core material before you do anything else. A messy, uneven hole is a nightmare to fix compared to a clean square. Take a utility knife and clear out the loose gypsum and torn paper until the edges are solid. If you leave the crumbling bits behind, your patch will eventually just fall out of the wall.
For small dents or holes from a doorknob, grab a self-adhesive mesh patch. Stick it over the gap pressing down with your taping knife to make sure it sticks to the drywall. Next, spread a layer of joint compound over the top with a putty knife, you dont want it too thick just enough so the patch is covered. It really is that simple. Just make sure the mud covers the mesh entirely so the grid pattern doesn't show through your paint later.
Wait for the compound to dry completely. Next scrape off any high ridges and apply another coat but this time using a bigger knife and wipe the excess off completely. Once dry a final skim coat should give you a nice flat repair without any pits or air pockets. After the compound is dry lightly sand the repair with a 120-240 grit sanding block. Don't rush this part. If you try to sand mud while it is still damp, you will just create a gummy mess and ruin the patch. Once it feels hard and chalky, use a 120-240 grit sanding sponge to smooth it out. Rub it until the edges of the repair blend into the surrounding wall.
Bigger holes require some actual structure. Cut a scrap piece of wood slightly longer than the hole is wide to act as a backing. Slip it behind the drywall and hold tight to the back side of the drywall while you drive screws through the existing drywall and into the wood. Now you have a solid surface to screw a new piece of sheetrock into. Tape the seams and you are ready for mud.
Apply your joint compound in thin layers. The first coat fills the gaps. The second coat hides the tape. A third coat usually makes the repair disappear. It is much easier to add another thin layer later than it is to sand off a giant mountain of dried plaster because you got impatient.
We are here at MrWalls Drywall & Painting to provide a free quote if you decide to hire a Pro. (413)302-0640