1) Google up a good "overhead" view of a galaxy. Something like this, this, or this.
2) Swipe that pic and open it up in a paint program. Any paint program will do, including MS Paint, which I'm using for my example.
3) Draw some squiggly freehand lines all over the map in some garish color. Don't get too out of control here. You want maybe 10 to 20 discrete areas on the map when you're done.
4) Type a number or letter into each area.
5) Create a key giving some sort of nifty label to each area of your galaxy.
6) Share the picture and key to your own custom galaxy! Like this:
7) Next time your players ask to see your campaign map, hand them a printout of your galaxy!
A- Starclusters of the Exiloids
B- Unincorporated Worlds (per 3rd Treaty of Arethon IV)
C- Reorganized Methanic Empire
D- The Multitude of Microstates (all 10,000 planets or less)
E- Realm of the Cosmokhan
F- Barbarian States (overrun Imperial Methanic colonies)
G- Territory of the Transentient Energy Field
H- Omniprosperity League
I- Unrestricted Trade Region (a vast warzone)
J- Robo-Sun Collective
K- Alliance of Unaligned Nonallies (the name makes more sense in the original Drakthani)
L- Tricameral Sophont Pact
M- The Ruined Worlds
N- Transgalactic Colonies of Humanity
O- Hyperclone Dominion
P- There be dragons here.
Q- Also unexplored.
R- Original Methanic Empire
Extra credit for Traveller fans: Overlay an 8 x 10 numbered hex grid on your map and develop a Universal Galactic Profile system that allows you to provide useful information for each hex. Keep in mind that the 3rd Imperium and all its neighbors comfortably fit in a single galactic hexagon with lots of room to spare.