Supplementing

Hi! Im looking to supplement with formula while at work but im not sure how to go about doing that? How did you ladies find the right formula for your baby? What types of thing did you look for in the formula? How did you know it was the right one? What should i be looking out for when starting my baby on a new formula? If i decide to switch formulas, how should i transition her from one to the other? Cold turkey or slowly? Thanks in advance! So far we started off with similac advanced just because we were desperate but i think it may be causing her some tummy trouble (constipation)