Yes, PeterAM, welcome to the Grumble! Be prepared, you will get a multitude of answers on this topic. I believe most of them will be "correct", based on personal preferences of each framer. The last thread on this topic nearly caught fire!
I've used good ol' spring clips for many years, spaced evenly and closely enough for even pressure, otherwise buckling may happen, the art backed by foamcore or topped with matboard if you really want to get pretty, as Steven6095 suggested. I use a small matbd scrap under the clip to avoid the metallic "skid marks" they'll leave otherwise, then pull it out when it's under the frame lip.
Some find eye protection necessary when using spring clips (BOING!), some will not use them at all but use enough layers of backing/matbd to squeeze the package into the frame.
Ultimately, you'll have to decide what's best for you. Fasten your seatbelt!
You might look under SEARCH at the top of the page and get pages and pages of "suggestions", if you need your answer right away, considering it's Sunday and sometimes slow here, many are out there doing non-framing stuff.