The first thing to go on your tree is your tree topper. Use one made of plastic, resistant to weather damage. Position your ladder next to the tree and find the center stem; if it's too long, clip it with your clippers. If it's too short, clip the surrounding branches until you have enough stem for your tree topper to sit on.
Next come the lights. Use only lights certified for outdoor use. Many tree toppers have lights in them; if yours does, connect the first strand of lights to the tree topper and then start stringing the strand around the tree, clockwise or counterclockwise. Otherwise, just wrap the end of the light string without the plug around the top stem, just below the tree topper, and then proceed with stringing your lights. Work your way downward, wrapping the lights around every major branch about 2 inches from the tip.
Next comes the garland, which again begins at the very top. Use a thin bead garland, which is more suited to rain, dew or even frost than a thicker garland made of cloth. Follow the light trail, swagging the garland from branch to branch.
Your final touch: the ornaments. Use ornaments specifically made for outdoor use, like hollow plastic balls, stars, icicles and other shapes. You may also use pine cones, perhaps painted with glitter paint to make them a little sparkly. Save the glass balls and homemade ornaments for indoors. Mix big shapes with small ones so the tree doesn't appear lopsided, and feel free to readjust after you've finished and you've had the chance to step back and take a look at what you've done.