During each of the phases of this operation, have a persistent (or semi-persistent, see details on that below) message that tells the user what's going on.
Before the operation display (alter the tone to match your brand):
Here's how we measure your yard:
1. First, outline your entire property in green with the pen tool (mentioning the color and the shape is helpful - multiple sensory anchors help the sense of solidity)
2. Then, outline the parts that aren't grass in red with the scissors tool
When they're in Phase 1, display:
Step 1. Outline your entire property in green using the pen tool
When they're in Pase 2, display:
Step 2. Outline any non-grass areas in red using the scissors tool
(These could be shortened too: "Step 1: Outline your property", "Step 2: Outline non-grass areas to subtract them"
A few things to note about the wording:
- In the full sequence "primer" before their operation, I don't use the word "Step" because when the full sequence is being viewed it's obvious it's a sequence.
- In the instructions displayed during the operation, I've removed the words "First" and "Then". Those words make sense when considering the sequence as a whole. During the operation the word would just be "Now" which doesn't need to be stated.
What I mean by semi-persistence: Something similar to your controls that appear or disappear with a tap. Maybe the current-step instructions appear with an x so they can dismiss it, or they fade in after a period of inactivity.
Finally, you can add some tiny green words inside the colored regions to remind them what the regions represent.
During Step 1: green text inside the green region that says "Your property"
During Step 2: green text inside green region that says "Grass". Red text inside the red region that says "Not grass".