The basic formula for square and rectangle pieces is:
finished size (the part that shows after the seams are sewn) plus 1/2 inch.
Example: If the finished size is 3 x 6 inches, cut a piece 3-1/2 x 6-1/2 inches.
When it comes to triangles, the numbers are different. One needs to add more for the non-right angle points than 1/2 inch.
Part of it depends on how you are cutting the pieces.