For the recipes that direct you to simmer a liquid, the key is to get the liquid to a simmer and keep it there until you accomplish the recipe's intent. How you get it there is really not important to the process.
All that's really important is to get it to a simmer without letting it boil furiously because you walked away.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan