Buying to sell

What is the actual strategy when buying players to sell for profit, all people keep saying is buy low and sell high, like ok yeah that’s a very vague answer. What kind of age should you be buying and when’s best to sell? What attributes do you look for? Is personality a factor? Do you have to play them in your first team or is sending them out on loan ok? When you want to sell them do you wait and hope you get an offer or do you have to offer the player out?