Most weed trimmers have a head you tap on the ground to release more line when needed. Line that is not wound properly will get jammed and not release smoothly, causing delays and stretching a quick weed-trimming job into a lengthy project. It is much easier to string the line correctly in the first place, saving yourself the time and trouble of untangling or restringing.
Remove the center piece from the weed trimmer bottom by depressing the button (or buttons, depending on the model of trimmer) on the casing sides and pulling the bobbin straight out.
Push the end of a weed trimmer line refill through the hole in the side of the bobbin from the inside until a quarter inch or so is sticking out. Be sure the line used is the correct weight for the model of trimmer.
Place your forefinger on the line to hold it in place on the outside of the bobbin, and wind the line around the bobbin four or five times in the direction indicated by the arrow on the outside edge of the bobbin. Wind it from top to bottom, in even layers, taking care not to cross the lines.
Cut the line with a garden knife or shears, when the bobbin is filled to the outer edge. Do not overfill, as this could cause the bobbin to not fit in the casing correctly and the line to not release properly.
Push the end of line through the hole in the bobbin casing from the inside out, and pull it through until the end of the string will reach the metal guard plate (six to eight inches). Tap the bobbin head to test your restringing job; the line should release smoothly.