You need two parallels. They can be straight wood sticks about 1-1/2" high at least 3/4" wide and as long as the widest boards you will plane. Mine are 18" long. Set one parallel at each end of the board. Sight the parallels. If there is a cup or twist you will see it. The parallels will not look parallel in height One end will look higher than the other. Hand plane the high spots. Learn to use a smoothing plane and also how to hone the iron. A router sled type thing will also level the boards. Make sure the high spots have shims underneath so the board does not rock. This will require a card scraper or sanding to get the swirl marks from the router cutter.I've got a teeny tiny little shop and am in the early days of kitting it out. We've got a table saw, a mitre saw, an orbital sander, and some hand tools. I don't know that we're ever going to have room for a jointer, and while I'd love a benchtop planer we don't have that at the moment either.
I'm doing a bunch of glue-ups of fancy patterned hardwood, but need to get them perfectly flat — how would you smooth out your stock if you didn't have a jointer or planer? (Honestly the idea of using a hand plane on them makes me feel a little ill, but I'm open to learning how to do that if that's the best/only way! 😅)