Alternatives to Procreate
If you don't have an iPad - don't feel left out. Autodesk Sketchbook is a good alternative tool for using layers to check the accuracy of your drawing. You can use it on the iPad, too, if you don't feel like paying for Procreate.
Layers are on the righthand side. Highlight first layer and add your reference photo by tapping on the add photo icon (rectangle with mountains and a plus sign) at top. Navigate to your photo - choose it and enlarge it to fill the screen. Hit “done”
Add a layer by hitting the plus sign in the layers window. Tap the add photo icon again, but hit cancel on the photos screen. Choose the camera icon from the menu across the top.Hold the tablet directly above your sketch and take a picture. Click “use photo” if you are satisfied or “Retake” if you aren’t.
Your photo will now be on top of your reference photo. Click “Done”.
Tap on the photo you just took in the layers window. In the menu that appears, move the opacity slider to the left about halfway. You should now see your reference photo underneath your sketch photo.
You will now need to move your sketch photo to align it with the reference photo. Click the icon at the top that looks like 4 arrows pointing each direction. This will select just the layer that you are working with in the layers window. Move the layer around until the eyes line up with the eyes of the reference photo. You can see where you are “off” and correct your drawing on paper.
Now hide that second layer with the photo of your sketch by clicking on the little eyeball in the upper left corner of the layers window and follow step 2 to add a new layer with a photo of your revised sketch. Keep repeating the process till you have your drawing the way you want it.