Step 4 - Painting
I can't stress this enough. Buy good paint! Cheap paint is plentiful but it doesn't cost much more to buy quality, and you'll be thankful you did long-term.
Only buy paint that is designed to stick to plastics. I suggest the Krylon Fusion "camouflage" series; it's the best paint i've ever used for this job. It costs a little bit more than the cheap stuff, but trust me - it's totally worth it!
Before you proceed, check your containers and make sure they're completely dry. Be sure to check around the edges and the groove where the silicone seal is. Even a drop of water can cause some major headaches and ruin an otherwise stellar paint job.
Also by this point you should have selected an area suitable for painting. Naturally it's not a wise idea to paint near a house or anything that you don't want to get paint on. In addition, you'll want to wear old clothes, as it's virtually inevitable that you'll get some paint on yourself. It's also probably not a wise choice to paint on a windy day - spray paint can travel remarkably far from where you're working and it doesn't clean up easily.
So now it's time to paint! First and foremost, triple check to be absolutely sure that your containers are completely bone-dry. If possible, let then sit overnight in a warm area before beginning this step. Be careful when handling them and be sure you don't get any fingerprints on the surface - this can cause the paint to adhere weakly and possibly flake off later. If you're concerned about handling them prior to painting, you can always give them a quick wipe-down with some high-strength Isopropyl alcohol first.
Be sure to follow the directions on your paint, most manufacturers don't recommend painting when the temperature is below 10°C or above 40°C. It's also not a good idea to paint when it's windy or raining, for obvious reasons!
Working outdoors, spread some newspaper or cardboard down when you want to do your painting, making sure that you can freely move 360 degrees around your work. Paint in even strips, holding the can on a slight downward angle about 15 cm from your target, working from one side across the container. Then move 90 degrees and touch up any missed spots, continuing until you have placed a light coating over the entire surface - don't put too much paint on any one spot, you can always come back and add more later. Be sure to pay particular attention to corners and latches, as these areas tend to get missed.
Let the containers dry for about an hour before applying a second coat. I like to hold the container up to the light to identify any underpainted areas. After letting the second coat dry for at least an hour, move your finished containers into a dry well-ventilated but protected area, such as your garage, and let them sit for 24 hours to completely dry.
So there you go. It's a little bit of extra work but your cache will have that personalized touch that geocachers will remember (and thank) you for. If you have the time and the creative urge, feel free to embelish the container's paint job with camouflage patterns or text.