How do you effectively use Shurelya's?

I'm not very good at this item. I often just forget that it's there if I build it, but let's just pretend I practice using it so that I remember it's there and to use it lol

How do you make the most out of it? What are you looking for in a fight to know that 'this is the moment'? Do you tend to use it to engage or hold it to retreat if needed?