There are two ways to mark a Step as complete.

The Quick Way

Step 1 – Display the Goal list.
Step 2 – Click in the arrow at the bottom of the Goal that has the Step you wish to complete

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Step 3 – Click the circle to mark as complete.

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The Long Way

Step 1 – Tap on the Goal the Step belongs too.
Step 2 – Select the Steps tab at the top of the screen.

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Step 3 – Select the Step you wish to mark as complete.
Step 4 – Press the Completed button in the Step Status Field.

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Step 5 – Reward yourself for completing a Step towards your Goal.