First step is to take body measurement as presented on this page. You’ll have to values: B (under bust girth) and C (under-wire size).
Note: these bras are for standard breast shape and volume. If you know to have a particular shape, bare in mind, that my bra pattern is a starting point. You will have to adapt the pattern according to your shape. Special articles on breast shapes, finding the size and the breast shape and pattern adjustments will be provided.