Graffiti is not funny.
Its not a joke.
Extinguishers are not fun.
They are serious.
Using an extinguisher to paint a wall is an act of pure anti-social behavior.
So is telling someone how to do it.
Step 1: Get a Fire Extinguisher
First, find yourself an extinguisher. It cannot be a red extinguisher, it must be silver or chrome so that you can see your stupid distorted reflection on the side of it. It also can be identified by the "bicycle tire" valve on the top near the handle/trigger. You can often find these in industrial buildings and large school buildings.
Step 2: Empty your Extinguisher
Now, spray out the funky aged juices from within the extinguisher. Make sure it is empty and out of pressure before opening. Open using a large wrench.
Step 3: Fill the Extinguisher
Get a bucket of water based paint. Decide what color to use. I recommend finding something that is dark or that is very light. Humanoids are likely to choose colors close to neutral gray to use on their homes and businesses to keep things calm. Go For Contrast. MAKE A FUNNEL and pour 3/4 of the paint into the empty extinguisher. fill a full bucket with water, AND USING THE FUNNEL,pour into extinguisher now containing paint. Close the beast aka seal the chamber. Tighten the top nut and shake vigorously.
Step 4: Pressurize the Extinguisher
Take your filled organ to a gas station and get some change. You can pressurize an extinguisher with any sized pump you want. I've fully charged one using a dinky fuckin nike bike pump that was shaped kinda like a vibrator. But gas station air compressors are the quickest and are often manned by overworked immigrants who are barely conscious. Fill the extinguisher with air. You'll feel the paint begin to severely grumble and gurgle. This acts to mix the paint and water once again. Keep the pressure on for approximately 3min. I often will often fill it with air until the compressor turns off.