Choose the color you would like to obtain. Do not choose primary colors, because these cannot really be created by mixing two other colors together.
Squirt from the paint tubes the two colors used to make the one you have chosen. Note that purple will be made with red and blue, green will be made with blue and yellow, and orange will be created with red and yellow. Make two separate nickel-sized piles of paint on your palette.
Dip your paintbrush into the lighter of the two colors you squirted onto the palette. For example, if you are mixing orange, you will be using red and yellow paint. Dip your paintbrush in the yellow paint.
Paint a large circle in the middle of your palette with the paint you just picked up with your paintbrush. Fill in the circle.
Dip your paintbrush very lightly in the darker color on your palette, so that you pick up only a tiny bit of paint. Note that any time you are adding a darker color to a lighter color, you should add the darker paint by tiny amounts, to avoid going too dark.
Cover the circle in your palette with the darker color you just put on your brush. Mix the original color and the new color together.
Examine the color you just created. If it is too dark, dip the paintbrush in the lighter color you squeezed from the tube and add it to the circle. If it is not dark enough, dip your paintbrush in the darker color and add it to the circle.
Do not add white or black unless absolutely necessary. If you do add white or black, mix it with only a small portion of your color as a test, leaving most of the color untainted.