Find the shape
A stud count matches every shape with that many studs; a size like 3x2 is far more specific. Add keywords to narrow further — 2x2 dome, 2x2 axle, 3x2 pin holes — or search by keyword alone. Printed versions come after you pick the shape.
Nothing with that description
Try a stud count like 8, a size like 2x4, a keyword like dome, or the two together.
Start with one brick
Count the studs on any piece in the bag and type the number, or just describe it. Pick the shape, then its colour.
Pick the colour
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…or a printed version
Decorated pieces are the most telling — many appear in only one set. Nearly all exist in a single colour, so picking one adds it straight to the bag.