Find yourself a large flat stable surface that you can attach some 80 grit PSA paper on, then simply run the block/board over the sandpaper. I have a large piece of alluminum plate and also a glass tabletop that works well. You can use anything flat and stable.
Also keep in mind, some of the best blocks and boards can be made yourself with wood, or alluminum, or plastic, etc... Example: many of my color sanding blocks and sticks are made out of 1/4" and 3/8" plexiglass.
Also remember to slightly round off the edges and corners-this helps you avoid any digging action if you slip up on a stroke.
Your work will only be as straight as what it was sanded with, and it's truly amazing how perfect you can make something when you get into the finer grits on a straight flat block, each step down in grit uncovers smaller imperfections not seen during the last stage of sanding and this process goes all the way down to colorsanding before buffing.
Watch those boards with wooden handles-the wood absorbs moisture/shrinks/swells and will warp with age. Also watch any boards with vinyl padding-when the adhesive on the vinyl starts coming loose it'll curl and collect sanding debris between the alluminum and pad which makes an uneven surface. Rubber blocks shrink and warp. Plastic boards like the 3M yellow and hustchins white will shrink and warp. Alluminum boards often get dropped on the corners and deform or the screws holding the handles back out. Bottom line is pay attention and it'll pay off.
Working in the shops I always heard technicians complain about how this block doesn't work well, or I can't make anything straight with this board, and this one works better than any.... problem is they weren't smart enough to figure out why. Ramble, ramble, Bob