Pace out a real garden. Sketch it on paper to scale. Calculate the area. You've just done the maths a landscaper does every day.
Try it
A rectangular plot is 12 paces by 8 paces. Your pace is 0.75 m. What's the area?
12 × 8 = 96 paces². Each pace² is 0.75 × 0.75 = 0.5625 m². So 96 × 0.5625 ≈ 54 m².
For weird shapes, divide into rectangles and triangles. Add the areas. This is exactly how surveyors did it for centuries.