Time to raid the recycling bin!
This is a little trick I use when I don't have enough pots for relatively small plants. The single serving soda bottles are perfect for seedlings and the two-liters are great for something a little more substantial. Since my husband has a Diet Dr. Pepper addiction, I have a nearly endless supply of the two-liters....
Get a hold of your bottle, remove the label, and rinse it out. Now that it's clean, the first major step is to slice the bottle roughly in half. It'll take you a couple tries before you know just how deep you want each "half" to be. I usually cut mine just a bit below the half-way mark (as shown below), making the top part (which will hold the soil) as deep as possible. As long as the top half sits will in the bottom half, just go with whatever variation suits your purposes.
The most attractive way to cut the bottle in half is to pick your spot, turn it on it's side, and saw through it carefully. You'll need either a fine-toothed saw or a very large kitchen knife you don't care too much about. Another option, though a less attractive one, is to hack at the bottle with kitchen shears. That's what I did this time around, since these pots are temporary anyway. To get it started, you may need to pinch part of the bottle and snip it, then unpinch it and start cutting your way around.
Now you should have the makings of a soda bottle pot before you! If the top inverts into the bottom as you like, let's start thinking about drainage. Leaving the cap on won't allow any drainage at all, but taking it off will mean all of your soil washing into the bottom. The solution is to put some holes in the cap. If you have a drill handy, use that. If you don't, then here's what I do when I don't feel like digging out the drill for small jobs. Take a nail and hold the end of it in a flame with a pair of pliers for about 15 or 20 seconds. The blue part of the flame is the hottest, so aim for that.
Now, still using the pliers, quickly but carefully press the tip of the hot nail through the cap as shown below. If your nail is hot enough, it should melt right through with just a little pressure. Three holes should do it, but you'll need to reheat the nail between each hole. Please make sure you're in a well ventilated area so you're not breathing chemicals from the melting plastic.
Put that cap back on the bottle, invert the top into the bottom, fill with soil, and plant! This one is pretty ugly. It was a rush job, just to get the baby strawberry plant into some soil. You can cut it much more nicely and even decorate it if the pot will be used long-term.
When you're done with the pot, please don't forget to recycle it!!!