Break the irregular shape into smaller regular shapes
Measure the dimensions of each smaller shape
Calculate the area of each smaller shape using the correct formula
Add the areas of all the smaller shapes together
If the shape has missing parts, calculate the area of the larger enclosing shape and subtract the missing area
If needed, use a grid and count full squares and estimate partial squares
For curved or very complex shapes, use coordinate methods, graph paper, or a planimeter
Check that all measurements use the same units
Write the final area in square units