Should I simmer or boil and for how long?

Doesn't really make much of a difference whether you boil or simmer, IME. You want to let it go for at least 25 mins. People say 45 mins is ideal, but I'm not patient enough for that so I'm not sure :)

Add lemon juice prior to heating?

I do. I add about 1-2 teaspoons of lemon juice to the water prior to heating.

What is the best way to filter?

For powder, a t-shirt works best and is also much faster than a coffee filter, which will take several minutes or longer to strain. When I make very large batches of tea (like for kratom kombucha), I got one of these specifically for straining kratom tea: https://www.amazon.com/gp/product/B01HIKU16M