Here's my current thinking on how I will approach Aquinas.
First I will study Summa Theologiae: A Concise Translation. This will take me 2-3 years. It's quite a bit of time, but it's time well spent.
After that, I will probably study Brian Davies's exposition of Aquinas – The Thought of Thomas Aquinas – to correct misconceptions I have about Aquinas' thought.
From there, I might go on to studying Aquinas: Selected Philosophical Writings. Or I may go back to the Summa, in its unabridged form, in a recent translation, e.g., the Hackett Aquinas series.