I have been cutting my own hair since 2001 and wanted to share a few suggestions.
First let's begin with the items you'll need to do the job correctly.
a)Good quality Wahl American made clippers, the ones with colored guards are preferred
b)Wall mounted extension mirror (with a standard and magnifying side) installed adjacent to a larger medicine cabinet, but far enough away that you can see the back of your head in the reflection off of the medicine cabinet mirror - also obtain a handheld mirror
c)make sure you have decent dust pan / whisk broom or vacuum handy
You will get hair everywhere, but so long as you have a hard surface floor - no worries, cleanup when you are done is quick.
Cut your hair in the bathroom, preferably in the mid morning after you have eaten. You won't be fatigued from hunger or sleepy. Plus the lighting will be better. Clothing is optional, or you can wear a smock - don't wear socks unless you like to waste time cleaning things.
Unravel the trimmer cord, hold it from the plug end and let it spin till the kinks are out, having a smooth cord makes for a more enjoyable hair cutting experience.
Always start guard-less, with the adjuster just off the end so you don’t hurt your skin. Stand in front of the mirror faced away and use either the pocket mirror or the wall mounted mirror to shape your rear hairline profile. Next trim your sideburns and ear line. (Being a little ambidextrous is handy but not completely required.)
Next add on the #1 guard = 1/8" length to the trimmer. Make sure that the oscillating blades don't contact the guard, as otherwise it makes an annoying noise. Trim against the grain first (up from back and up to your crown) brush your head a few times to ensure that seated hairs can't escape the clipper on your second pass. Repeat with the sides your head, and again go against the grain.
Check the back of your melon in the mirror a few times to make sure you don't have any freakish patches left over like 'Sloth' from Goonies. also if you want to leave a little more hair on top (if you have any) use a #2 or #3 and fade in the line between the sides and top.
Once your done with your new skin junk, optional manscaping can follow - but you're on your own for guidance in that arena. When you are cleaning up the floor, put the cousin "It" hair clump that you have just generated into a trash can where it's not staring at the next bathroom user.
When you shower afterwards, make sure to really shampoo well and massage the scalp thoroughly, it will help to reduce ingrown hairs. also, check your ears twice - little hair clippings always seem to loiter there for awhile after a haircut.
After you cut your own hair a few times you will learn what works best and have it down to about a 10 minute operation. Good luck!