People here say to spend a day being sedentary in Denver or someplace with a similar altitude.

Then start with an easy hike at around 6 to 7,000 feet.

If you are in good shape, acclimation might take longer. It's highly individual, so be flexible.

I still have trouble above 11,000 feet even though I go high a lot. The best month is early August.(Big differences of opinion here.) In late July, you should be able to hike up to some snow patches around 11,000 feet, but that's highly variable in time and location.