Baseball cards, especially older ones, are prone to curling over time due to changes in humidity and temperature. Properly flattening curled cards is important to preserve their condition and value long-term.

The first step is to remove any surface dirt or debris from the cards. Use a soft brush or microfiber cloth to gently sweep away any dust or particles stuck to the surface without scratching the cardboard. Thoroughly clean both sides so no grime gets pressed between the card and flattener later.

Once clean, the next step is to select the proper flattening material. Soft, pH-neutral material like paper or non-PVC plastic sheet protectors work best. Avoid using potentially acidic materials like cardboard that may damage the card surface over years. Cut the sheets slightly larger than the card dimensions to avoid creasing edges when placing weights on top.

Arrange multiple weight books or stacks of other heavy, flat objects on a stable, level surface like a table. Make sure there are no small shards or protrusions on the bottom surfaces that could scratch the card faces. Place a sheet protector or paper down first, then carefully insert the curled card centered face-down.

Cover the card completely with another sheet of the flattening material. The sheets should fully encapsulate the card edges to distribute pressure evenly across the entire surface when weights are added. Place additional sheets over any exposed card edges if needed.

Starting with lighter weights first closest to the card, gradually build up layers of books or other heavy objects on top of the flattening sheets in a symmetrical pattern. Place weights evenly across the entire surface without leaving gaps. The goal is to apply even, steady pressure across both sides without overly compressing the card thickness.

Leave the setup alone in a location away from direct heat or sunlight for at least 2 weeks up to a month for optimum results with severely curled cards. Check periodically and add/remove weights as needed if any curling remains. Slow, prolonged pressure over long periods works best rather than forcefully trying to flatten quickly.

Once straight, carefully peel back the top layer of flattening material. Inspect for any stray debris stuck to the sticky side and gently remove with a soft brush. Then slowly lift the card and check that it lies completely flat. If slightly warped, rearrange weights and leave a while longer. Fully flattened cards can now be safely stored, sleeved or displayed.

Proper flattening takes time but safely restores cards without risk of damaging surfaces. With patience and the right materials, even seriously curled vintage cards can be revived. Taking care during the process helps preserve fragile cardboard and images for enjoyment of future collections. Let me know if any part of the flattening process needs further explanation or details.