Rather than buy a lot of pots to put those in, or purchase a bunch of high-dollar planters for seed starting, I like to just collect containers throughout the year and then use them in the early spring to get my garden going. I live in the north-central U.S. (Wyoming), so getting a head start on my garden is important.
Until you start collecting them (I have a shelf in the basement that these end up getting stacked on), you never realize how many plastic containers of various shapes and sizes you end up with over a year's time. Dozens upon dozens if you have a fairly normal lifestyle. Hundreds of thousands if you're a soda addict.
Step 1: Cutting the Bottle to Size
First, take the bottle and select a spot roughly between halfway and 2/3 of the way up the bottle (in height). How high you decide to cut it is up to you, but the deeper your bottom portion is, the more dirt you'll need and the more roots you'll have for transplanting. As a rule of thumb, I cut them in half for seedlings to starters that will be transplanted and deeper if I plan to use it as a semi-permanent growing pot.
Press the bottle fairly flat and cut it with scissors. A knife will work too, but it's easy to slice yourself instead of the bottle, so I prefer scissors. I still have all ten digits on my hands, so that's proof in the puddin'.
Step 2: Holes for Drainage
The next photo shows that after it's been done. I was pretty uniform with my pattern, but you can go crazy if you want. Use a hand drill and small bit or an ice pick and hard surfaced table or workbench.
Step 3: Put in Gravel and Soil
Then fill the rest with potting soil. Nutrient-rich stuff can be used for seed starting or regular dirt with some nutrients added can also be used. The soil shown here is out of the bag, which I received in trade for helping a neighbor. I've also made good soil by mixing thirds of nutrient mix and dirt (1/3 mix, 2/3 dirt) in a bucket or wheelbarrow.
Step 4: Planting In, Using, and Enjoying Your Garden Containers!
The seed bed you can see in that background is the cheap type you buy from Wal-Mart. I had a couple of those that were given to me by someone who wasn't going to use them. The plastic is very cheap, flimsy, and breaks easily. Not recommended unless you need catcher trays for drainage.
Which brings me to the last point: catch pans. You'll need bottoms for these containers to drain into so the water doesn't just sieve right through or end up on your floor. The coffee cans have lids that work perfect for this. We used the trays you can see in the background from the seed starters as well as old cookie tins. Unused lids from Tupperware, Rubbermaid tubs, and a lot of other things can be recruited for this as well. I even used an old oil pan (thoroughly cleaned) once.
This is a great, low cost, and easy way to get your garden going from seed. Plus, it's a great Saturday project on the porch for the whole family to get involved in.