Partial shade for the rest of the day is acceptable. Because they thrive when the weather is cool, plant them in late winter or early spring or in the fall after the nights become cool. Probably, you should start with small plants which can be bought at garden centers, discount stores and other places which sell bedding plants. These plants are well established and have good root systems. The plants will be accompanied by tags which will have a picture of the variety and information on spacing, fertilizing and plant height.
Most commercial growers pinch the growing points on shoots.
This encourages branching. Don't plant snapdragons too deeply. Deep planting will encourage stem and root rot. Space the short varieties six to ten inches apart, taller varieties should be spaced at least 12 inches apart. Use two pounds for 100 square feet of area.