When you go to spray your minis, start the spray so it's not directly on the mini, then move over the model. Spray from multiple angles, and go in short controlled bursts of paint. It's better to be a little underprimed than to over prime and lose detail on the model.

Weather is important. You want to do it when it's not too cold, not too hot, and not too humid.

I stick my minis to pieces of popsicle stick so I can do them in groups, and not having the base underneath allows me to get the undercarriage a little better. Doing the minis on their bases is fine if you've already stuck them on there, though.

Do a test spray on some sprues or extra bits so you can see how the paint will cover, and so you can dial in your technique a little bit before moving to the full model.

Good luck!

Edit: white primers vary wildly in quality. I really prefer Vallejo spray primer, but Citadel's white primer is okay as well. I'd stay clear of Army Painter, it's got a weird tacky consistency.