The Art and Science of Sails by Tom Whidden has more than you want to know.
This website has the basic trimming guide.
http://www.ukhalsey.com/LearningCenter/encyclopedia/encyclopedia5b.aspEverybody has been telling you the right things, but in pieces.
Put two sets of telltales on your jib, one set high, one set low about 1/3 and 2/3s up. If your fore and aft position of the jib block is correct for upwind sailing, both sets of telltales will luff at the same time. Move the jib block forward a little and the sheet will pull more down than aft than before and tension the leech a little more than the foot. This will pull in the top of the sail more than the bottom and the top leeward telltale will show that the top of the sail is stalled (sheeted in too tight) relative to the bottom of the sail. On your boat moving the jib block all the way forward to the front crossbeam will be much worse. A boat with the jib block on the front crossbeam has a sail sized and cut such that the angle of the jib sheet is proper to that point. Yours is not.
Now turn a little downwind from the close hauled course we've been sailing and sheet out the jib a little. As soon as you sheet out a little, since there is more tension in the leech than the foot, the clew will rise and the top of the sail will be luffing relative to the bottom, so you will need to move the jib block forward if you want to keep perfect trim. As you sail a lower course the block will have to move forward and outboard to keep the entire sail from top to bottom trimmed correctly at the same time. For your best point of sail to go downwind, approximately a beam reach apparent wind, the sheet point is around the outer end of the crossbeam. So a barberhaul to effectively move your sheet point there is for sailing downwind.
John