I remember seeing expressions of guerrilla gardening in Philadelphia while growing up, always thought it was cool but not much beyond that.
More recently I think this;

1. The more adverse the situation, the greater the need for risky activism.

2. If something is beautiful, true and beneficial to others just do it.

3. Creativity plus strategy yields change.

4. Don't assume someone else will do it.

Do It Yourself (DIY) Guerrilla Gardening

1. Identify some local abandoned land. A curbside, a median, an empty lot... anywhere green love deprived.

2. Plan a mission. Rally some friends or go to GuerrillaGardeing.org and look for a group. Pick a date and time. Generally speaking you'll be better off after hours.

3. Buy the plants. Think strength. You'll want to go for something that can look after itself.

4. Bring some water. You'll want to give your new green friends a good start with lots to drink.

4. Make it happen. Go dig, plant, water and run. Host an after party or a meet-up at your favorite watering hole.

5. Repeat with opportunity.

Seed Bombs.

Seed bombing is the drive by shooting of guerilla gardening. You can buy a bag of 5 pods on Etsy for about $7. Each one yielding a colorful mix of florals.

SEED BOMBS by VisuaLingualFind lots more on the "How To Guerrilla Garden" subject in Richard Reynolds book, On Guerilla Gardening.