This is where the footing, or foundation, of your column will be. It's important that you build the footing down into the ground as opposed to on the surface so that your column is stable. The depth and dimensions of the hole will depend on several factors, like the size of your column, how much weight it will support, and the kind of ground you're building on. If you're not sure, do some research online or consult with a professional mason.  As a rule of thumb, the hole should be at least 12 inches (30 cm) deep. Make sure the hole is at least 4 inches (10 cm) longer and wider than your column will be so you have enough room to build the footing. Verify that the bottom of the hole you dug is level before you proceed. If it's not level, your brick column won't be either, so you'll want to level it first. Cut a wooden plank or stud into 4 pieces—1 piece for each side of the frame—and nail them together to make a square frame. Then, place the frame in the hole you dug so it's laying flat on the bottom of the hole. Since you'll be using the empty inside of the frame to make your footing, make sure the frame is 4 inches (10 cm) longer and wider than your column will be when measured from the inside edges of the wood and not the outside edges. For example, if you want to build a concrete column that's 24 by 24 inches (61 cm × 61 cm), you'd want to make a frame that's 28 by 28 inches (71 cm × 71 cm) when measured from the inside edges of the frame. and let it dry completely. Once you've poured the concrete, take a piece of wood and drag it across the surface of the frame to scrape off any excess concrete and smooth out the surface so it's level for your column. Then, wait for the concrete to dry.  How long it takes for the concrete to dry will depend on the type of concrete you use. Some concrete can take weeks to fully cure, while others are designed to dry in a few hours. If you want to speed up your project, try using a fast-setting concrete mix so you can pour the concrete and start working on your column all on the same day.  Alternatively if the column isn't load-bearing or very tall, you can later bricks and more in an area 18 inches (46 cm) deep and 4 inches (10 cm) wider than the column. Dry-laying your bricks is just arranging them on the footing in the pattern you'll be using without actually securing them to the footing with mortar. Simply take all the bricks you'll use for the first course (the first layer of your column) and lay them on top of the footing so the ends are touching and they form a square or rectangle. Don't put a brick in the center of the course since the center of your brick column will be hollow. Also, leave a 3⁄8 inch (0.95 cm) space between each neighboring brick so you can connect them with mortar later on.  When the bricks are centered on the footing, there should be 4 inches (10 cm) of footing extending off of each side. Use a pencil to trace all around the outer edge of the course and then along the inner edge of the course so you're left with 2 rectangles or squares, 1 inside of the other. This will give you an outline to use when you spread the first layer of mortar on the footing. When you reach a gap between the bricks, just draw a straight line across the gap to the next brick as if the gap wasn't there.
Dig a square, level hole where you want your brick column to go. Make a frame that's 4 inches (10 cm) longer and wider than the column will be. Fill the frame with concrete Remove the wooden frame and dry-lay the first course of bricks you'll be using. Trace the outer and inner edges of the course, then remove the bricks.