Choosing meter polarities:

Remember that when you annotate a wire with a "current definitional arrow" you are implicitly inserting a ammeter (current meter) into the circuit at that point. Your arrow shows the direction of current flow that will cause your meter to read a positive number. If you write a variable name by the arrow (e.g. i3), this means that the value of the variable will be positive when current flows in the direction of the arrow, and negative when current flows opposite to the arrow.

It is totally up to you how to draw the current definitional arrow. It is customary, if you can guess which way the current really will flow, to draw the arrow in the directon of the actual current flow, so that when you later solve for the associated variable it will turn out to have a positive value. However sometimes the current oscillates, and sometimes it's steady but you can't tell which direction it will flow, and sometimes you guess wrong, and sometimes you are just in a perverse mood and draw it backwards on purpose. No problem. It doesn't matter.

What does matter is that, once you have chosen the current definitional arrows, you must draw the voltmeter polarities consistently with the current definitional arrows. If you don't, the constitutive laws that you later use will be wrong - they will have sign errors. Here are the rules for assigning voltmeter polarity: