It's especially easy if you are keeping a pair of frames that you know fits you well.
What you need is the actual prescription (or the actual prescription numbers for your eyes), the measurement for the PD (pupillary distance). You then send that to the online operation. You then pick the exact lens you want (there are many many options: how thin, what kind of coating...etc).
On average you can expect to save around 40%, at least, just for the lens themselves.