NOTE: I'll be revising this video to reflect a change that was made in the App's latest update. For now - just note that to change the opacity of a layer - Go to "Layers", click the "N" on the layer you want to change, and move the slider.
This video describes how I use the Procreate App on my iPad to help me get an accurate drawing (on paper) for a portrait without gridding, tracing, or other more labor intensive methods. It lets you practice your freehand drawing skills, but helps you make sure your drawing is correct.