Place your snare drum and key in front of you, in a position that is easily reachable.
Using your key, begin to loosen each tension rod. Do so gradually, as you do not want one to be entirely loosened while others are very tight.
Once all tension rods are fully loosened, remove the top rim and drum head.
Clean the inside of your snare drum, including the rim.
Place new drum head on the rim of your snare drum. (I do not actually have a new head, so I'm just putting the old one back on.)
Place rim and tension rods back on your snare drum and begin to tighten the rods gradually.
Once finger-tight, use your key to tighten the tension rods to your desired tuning. Do so in a star pattern in order to produce even tension on the rim of the drum and get a more even tuning.
Tune the bottom head on your snare drum to your desired pitch. I like to tune the bottom head to mid-range so that it works well with the top head tuning.
Place any accessories you had back on the snare drum. I use a couple dampening tools and tension rod locks.
Play your drum! Test to make sure everything is tuned well, and sounds great. Repeat any of the previous steps regarding tuning as needed.