I started to get one of the green items (Healbane, almost always) in the early section and then get Stamina Mastery straight away, skipping Enchanter's Emblem and Spirit Lifesteal. (Sometimes I get Spirit Lifesteal, depends on the matchup).
Middle game items are very flexible in their buy order, get the purples you need earlier depending on the situatuion and get Dispel Magic/Silence Wave as needed. You may also get any green from the category below them.
I prioritize the top part of the late game items section, lower part includes some luxury/counter items.
At the end of the day, Apollo is an ability/mobility reliant hero, thus, the item philosophy is getting those that enhance his two strengths. He excels in lane, can demolish whole teams in the mid game, but falls a bit behind other characters in the late game. Learn the techs, master the movement, and witness perfection.