I've been using an app called "Drawing References" that makes this feedback step a LOT easier than the Photoshop/Procreate process described in the past couple videos. Just take a picture of your drawing and it gives you an adjustable transparent overlay you can compare with the reference. It also has built-in grayscale and blurring filters to help figure out shading. Totally worth the $5. I'm on Android, but I see it's available on the Apple App Store too.