What is your volume of images to be framed, what value do you put on your personal time, and how important is control over the final aspect ratio of your image to you?
Assuming you do a low volume, your personal time is important to you, and you don't care about aspect ratio, then your best option is a pre-made frame, pre-cut high-quality mat, and then you assemble it yourself. Downside is you are limited to standard window opening dimensions.
Assuming you do a low volume, personal time is important to you, and you care about aspect ratio, then your best option is a pre-made frame, custom cut high-quality mat done by someone else, and then you assemble it yourself. Downside is higher cost.
Assuming you do a medium volume and/or you don't care about personal time, and you care about aspect ratio, then your best option is to use pre-cut frames, buy a reasonable quality mat cutter (not the $99 Logan crap one that you always see!), learn how to cut your own mats, cut them, and assemble yourself.
Assuming you want the most expensive option, but that requires the least amount of your personal time, get a full custom frame job from a framing store. A good store will do an awesome job. For a slightly cheaper option if you don't value your time too highly, get a custom frame and custom glass from a framing supplier, cut your mat yourself, and assemble yourself.
Assuming you do high volume and have a consistent aspect ratio, pre-order a whole stack of pre-cut mats and frames from a mat supplier, like
www.framedestination.con. When an order comes in assemble yourself and sell.
I could go on but I'm sure others (like farmah) will chime in

Neil