A 15 foot ladder isn't much of a problem for one guy. Assemble the whole thing, then pick it up and stand under the platform. Drag the bottom feet as you pick it up and move the platform up the tree. It will take a few minutes but you can always stop and rest. Make sure you use a safety belt when strapping it to the tree.

A 20 foot ladder is easier with 2 guys because of the added weight and length. Again, use a safety belt and have your buddy hold it as it will bend a lot more than a 15 foot until you put the braces against the tree.

It is easier to set them up before the season and take them down when it's over. If you are trying to do it on the same day, you will be tired.