Adding Line to Weed Eater?

Hello

I am having difficulty understand how to add line to a weed eater.

It is an electric, .065 single line weed eater.

The spool has an arrow (a hint as to which direction to wind, correct?) and it has little slots on one side (to the hold the line?).

How much line do I place in the slot and is that the line that I place through the small hole where the spool goes on the weed eater?

Also, I bought a 50' replacement line. Do I have to cut it and place, say 10' or 25' in the spool or place all 50' on the spool?

Thank you very much.