!. Divide it into polygons that you do know the area of. This is called the area of a composite shape.
2. Transfer the polygon onto graph paper and count the squares. You can do this by tracing the polygon on something like tissue paper that you use for wrapping presents and cutting it out.
3. If it is too large to use (step 2), then carefully measure the sides and reduce it by a ratio that will get it down to a practical size. Artists use a special projector to do this.
Perhaps I'm leading you astray. Please post a specific problem if you have one. Maybe it can be solved just by using the first one. I've suggested.