Training and acclimatization help, but only to a point. In that process, you can figure out how fast you can sweat (I can sweat about 2 quarts/hour, for instance), and how fast you can absorb fluids (for me, that's only 1 quart/hour). Pre-hydrate, get on the bike, and start drinking.
When my stomach starts sloshing, it usually means I need more salt.
When all else seems to fail, stop at an air-conditioned convenience store and get a popsicle or slushy, and consume that while chilling inside. (Not sure how to do that while wearing your mask indoors!)
Good luck!