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You start with a layered Photoshop or Illustrator file imported as a composition. Each of the elements you want to reveal must be on a separate layer. Then You import as a composition retaining layer sizes. Nest you devise the right masking technique to reveal the elements of your drawing. This can be an animated stroke, an animated mask, or an animated gradient used as a track matte.
You must be familiar with masking, using track mattes, layers, and key framing masks. If you are new to AE start here: Basic Workflow