Can I use a “free action” at any time during my turn, or do I have to do it at the beginning?

You can take any of the “free actions” at any time during your turn: before you draw, in the middle of your plays, or after all your plays are done. In fact, your turn is not technically over until you have used, or declined to use those options. If you simply forget, however, and let the next person take their turn, your opponents are not required to point it out to you, and once the next person starts their turn, you’ve lost the option.
Related Rule(s)