Write the heading, "Schematic" on one piece of paper, and on another, "Parts List." Under "Parts List," write "LM837 op amp, 10K resistor and 4.7K resistor."
Choose whether you want to run the mixer from batteries or a wall plug. If you want to use batteries, add a battery clip or holder to your parts list. If you want to use AC power, for convenience, use a 9-volt DC "wall wart" power supply. Find one in an electronics catalog. You'll want one that supplies at least 100 milliamps of current. Add this to the parts list.
Determine how many inputs you'll want on your mixer. For example, if you're planning on mixing a microphone, guitar and keyboard, that's three, but add one or two extra to let the mixer accommodate other things. Determine the kinds of connectors you'll want on the mixer, as microphones use XLR or phone plugs, guitars and keyboards use phone plugs and other equipment uses RCA plugs. Determine what kind of output connector you want--RCA, XLR or phone plug. Add these items to your parts list.
Study the list of schematic symbols. The mixer uses only a few different kinds of parts--resistors, variable resistors and the IC itself, so it has a simple design.
Draw the right-pointing triangle symbol for an op amp on your "Schematic" page. Label the top input with a minus sign and the bottom input with a plus sign. Draw a line coming from the bottom (plus) input and the zigzag symbol for a resistor. Draw the resistor connecting to a signal ground point. Label it "4.7K ohms"
Draw a short horizontal line from the top (negative) input. Draw a vertical line going up, connecting to the short horizontal line. Draw as many short horizontal branches from the long vertical line as you have inputs. From each branch, draw a zigzag resistor symbol with one end starting at the branch and the other end connecting to ground. Label each of these resistors "10K ohm." Draw a variable resistor wiper symbol pointing to the center of the resistor, and extend this to an input connector symbol. Add one 10K variable resistor and input connector to your parts list for every input you decided on in step 3.
Draw a short horizontal line coming from the right-facing tip of the op amp triangle. Add an output connector symbol to the end of the line. Draw a vertical line coming up from the op amp output, then turn left 90 degrees and add a 10K resistor. Extend the line from the resistor so it runs back down to the line coming into the minus op amp input. This completes the mixer circuit.