Preparing Cedar door for restaining

I have recently been loaned a tin of cedar stain (the person wants it shortly). What preparation do I need to do to the door before restaining it? Do I need to sand it? Have heard different views on this.
The current stain on the door is not too bad, but I would like to take up the offer of the free stain (may be slightly different shade). Does time of year matter (for drying etc)?
I am a complete amateur, so assume I know nothing (which is pretty much true).
Any help is appreciated,
Regards Scott

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