Aspen, Colorado, is in United States Department of Agriculture (USDA) Hardiness zone 4. Aspen's elevation is 7,908 feet. The treeline in the Colorado Rockies, the elevation above which tall trees don't grow, is 11,500 feet. Therefore, any zone 4 trees should grow well in Aspen.
Select a tree that will grow in the area. Aspens and a number of pines, spruce, and junipers grow exceptionally well. However, any zone 4 tree should do well, although some will need irrigation.
In the spring, after the risk of hard frost has past, dig a hole the same depth as your nursery tree's root ball. Dig the hole slightly wider than the width of the root ball.
Remove any plastic wrapping on the nursery tree. Some trees come with their root balls wrapped in plastic. Others come with the root balls wrapped in burlap. If yours is wrapped in burlap, leave it in place. It will decompose in the soil and provide nutrients to your tree.
Place the tree in the hole.
Look at your tree from all angles. If it is leaning to one side, add some soil on that side to help support the tree in a straight up-and-down orientation.
Fill the hole once you are satisfied with the placement of the tree.
Pack the soil down and add as much as possible in the hole. This will keep the tree from shifting.
Water the tree thoroughly. Use a soaker hose to prevent evaporation and overspray.
Mulch the area around the tree to prevent water evaporation. Mulch can also protect the roots if there is an unexpected freeze.