Intro - Checklist - Prep - Wall Prep - Repairs - Apply - Cutting In - Large Surfaces - Trim
Large Holes
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Large holes in the wall will need special attention. The repair procedures differ depending on whether it is an older wall with lath and plaster, or a newer wall using gypsum wallboard (drywall).
Repairing holes in lath and plaster usually involves several steps. First, clean the hole and the edges of the hole of any debris. The sharp end of a can opener comes in handy here. If the lath is still intact, you can start to fill the hole with compound. If the lath is missing or badly damaged, you will need to stuff something into the hole to serve as a backing for the drywall compound that will fill the hole. You can either use steel wool or a wad of newspaper. 
Also, a wire mesh held in place with a piece of string, attached through the mesh and around a pencil, on the outside of the wall will work. Place the newspaper or steel wool so that it is recessed about 1" from the finished surface of the wall.
Moisten the edges of the hole with a little water. Using a drywall knife that is at least 1" wider than the hole, spread the compound over the hole. Do this until the compound is about 1/4" recessed from the finished surface of the wall. Allow this coat to dry until it is tacky. Score this tacky compound with a nail to rough it up so that it will receive the second layer. Let this scored layer dry, then moisten and repeat the process, filling to within 1/8" of the finished surface. (Two coats can be used if the hole is less than 4".)
Sand this coat and apply the final coat, sanding this smooth with steel wool or a fine-grit sandpaper (100-grit silicon carbide). Use an orbital sander if you have one. To quicken the drying time between coats, direct a fan at the patch. Also, fast-drying compounds are available. Always clean your tools immediately after using this type of compound. And be sure to prime any fresh compound after repairing, but before painting.
An alternate method of repairing large holes in either lath and plaster or drywall is to use the "hat patch" method. This involves cutting a patch that will fit over the hole. First undercut the edges around the hole so that you have a good dean hole. Again, you can use the sharp end of a can opener. Around the perimeter of the hole tear away a 1" strip of drywall paper so that only the bare gypsum is showing. Use a utility knife to score the drywall paper around the diameter of the cut. This will assure that the hat patch will lie flat with no raised edges.
Cut a piece of drywall to the exact shape and dimensions as the total area defined by the bare gypsum. Remove enough of the gypsum from this piece so that you are left with a plug the size of the hole and a paper brim that will cover the bare gypsum. Apply compound around the hole and insert the patch. Cover the patch with compound, allow it to dry, and sand it smooth after drying. Apply a second coat (and third coat if needed), sanding after each coat Paint the fresh compound with primer before painting or papering.
Repairing Cracks
Cracks are usually rather simple to repair. Small hairline cracks can be repaired by simply spreading compound over them and sanding them smooth. Larger cracks need more attention.
To repair larger cracks, you first need to scrape and widen them. Also you need to undercut the edges. This can be done with a widening tool or the sharp edge of a can opener. Be sure all the debris is cleaned away. You may want to vacuum out all the dust as dust can cause adhering problems. After you have prepared the crack, dampen the edges before applying spackling or drywall compound. If the crack is large enough, over 1/8"-1/4", you should apply a self-adhering fiberglass drywall tape directly to the wall before applying the compound, but after filling the crack with compound. This will assure that the crack does not reappear. Apply two coats of compound with a putty knife, allowing the first coat to dry, then sand and apply the second coat, feathering the edges.
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100 grit is not fine sandpaper. - 220 is more like fine sand paper. 100 grit is used for really tough sanding and usually followed up by 150, 180, or 220 sandpaper. 100 will leave very visable marks underneath wall paint even.