Here is a fail proof way to decorate your tree:
First, string the lights so that you use a lot of lights. No need to go all around the tree if it is up against the wall. You can zig zag the lights back and forth, starting from the top.
If you are using garland, equally divide the space between each row of garland so that it is symmetrical. Much like the same zig zag pattern you used for the lights, but not as dense as you want more lights than garland.
Separate your ornaments into size categories. Small, Medium and Large.
The trick to hanging ornaments is that they hang freely and are not touching the branches below them. Also, it is visually pleasing to place ornaments deeper within the branches to hang there. This gives the tree depth and interest.
You will use the small ornaments at the top of the tree. The medium size ornaments in the mid section of the tree and the largest ornaments at the bottom of the tree. This creates a decorated tree with visual balance that is the key to cohesive decorating. Since the top of the tree is smaller, that is why smaller ornaments look good on top and when increased size of the ornaments as you decorate to the lower branches causes visual balance.
I hope this has been helpful. Merry Christmas to you and yours.