What are some good tips to do stretches/activities at home to help keep my heels down for when riding

Calf raises. It also helps to strengthen your upper leg and core (generally develop your position), so squats and planks!
No stirrups :)
If you have stairs, only put your toes on and just sink into your heels, don't bounce when you do it, stand there for a few minutes, or however long you can last and then take a break, repeat whenever
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