Put on gloves. You will be dealing with potentially sharp or hot moving engine parts, so be sure to don appropriate protective handgear before getting to work.
Start your Craftsman lawn mower and allow it to run for approximately two minutes. This will give the oil a chance to warm up a bit and will make it easier to drain. After turning off the engine, allow it to cool for approximately one minute.
Remove the spark plug wire from the spark plug. This will remove any chance of accidentally shocking yourself or unintentionally starting the engine while you are working on your Craftsman lawn mower.
Determine if your model of Craftsman lawn mower has a drain plug. Some models, primarily the riding mowers, have a drain plug at the base of the engine. For others, including most of the push mower varieties, it will be necessary to open the oil cap and turn the mower over to allow for oil drainage.
Place an oil funnel under the drain plug or oil cap of your Craftsman lawn mower, with the large end toward the lawn mower and the narrow end in a container of some form. An empty milk jug or empty coffee can works well as a container to collect the spent oil.
Remove the drain plug or oil cap on your Craftsman lawn mower. For lawn mowers with a drain plug, gravity will cause the oil to sluice down through the oil funnel. For push mowers, you must tip the lawn mower over. Tip the lawn mower away from the side where the air filter rests. This will reduce the chance of oil getting on the air filter.
Allow the oil to drain for approximately one minute, or until the flow of oil has been reduced to a drip or two every few seconds. Some of the old oil will still remain in the engine, clinging to the internal parts--but that is fine. As long as the majority of the spent oil is removed, your Craftsman lawn mower will be fine.
Replace the drain plug or right your push mower. If you are dealing with a mower with a drain plug, make sure to replace it at this point. If you are dealing with a push mower, turn it back to its normal, upright position.
Check your owner's manual to see what type of motor oil is recommended for your Craftsman lawn mower. In most cases, this will be SAE 30 4-cycle small engine oil--but it does vary on occasion by model. Always use the suggested oil for optimal performance.
Fill your oil tank until the dipstick registers "full". It is suggested to start by adding 10 ounces of oil initially and then checking the the dipstick. Pull the dipstick out, wipe it down with a paper towel, replace the dipstick, pull the dipstick out again, and then see where the oil level is registering. From that starting point, you can add oil in two- to three-ounce increments until the dipstick registers as being full.
Replace the oil cap and start up your Craftsman lawn mower. All the mower to run for at least two minutes in order to circulate the oil throughout the engine and to burn off any errant oil drops that might have spilled.