I've encountered several riders over the last few weeks who had flatted and either 1) did not have a flat kit, 2) had a flat kit but had used their tube and/or CO2 previously, or, 3) had everything they needed but didn't know quite how to execute the repair.
I'll address the situations in order with yet more unsolicited advice:
1) With cycling events and the long rides of summer upon us, no road rider should be without a flat repair kit. That's true even if you're just cruising the trail, but vital if you're going to head out on the road. Somebody will inevitably stop to help you, and it's really appreciated if have your own stuff and you can use the opportunity to learn from your good Samaritan.
It is an optical illusion that the bag looks as big as Willie. It is quite compact.
I stopped to repair a flat for a couple of women a week or two ago. They had apparently been equipped with a tube and a CO2 between them, which they had used the day before.If you have never had a flat, it's easy to be confident with minimal gear. I gave them a tube and used one of my CO2s to get them going. I also suggested that when they visited their local bike shop to restock their kit that they buy at least two of everything.
For about 25 or 30 bucks you can stock some ride insurance. It's not bad policy to have a tire on hand, as well. I'd rather but this stuff at my convenience that be rushing to the bike shop at closing time.
3) Local bike shops and groups sometimes do clinics on changing flats and making minor repairs, but it is pretty easily learned. If you are not really confident in your abilities, practice at home. Pull up a video on YouTube, let the air out of a tire, and practice. Use your floor pump to re-inflate, but be certain that you know how your inflator works. Most instructions consist of: screw in CO2 cartridge, seat inflator on stem, back out CO2 slightly to inflate.
Read the directions.
I post something similar to this article at least once a year, but some things are worth repeating. I'm sure that Diane will tell you that this article is better than some of my stories that she has heard too many times over the years.