Black & Decker manufactures a number of corded and cordless string trimmer models, including one with a lawn mower-type chassis that you can use to cut the grass. Because these trimmers are intended for light-duty residential weed trimming, lawn cutting and edging, they accept only 0.065-inch string, which is one of the thinnest-diameter microfilament strings available. Most models have spring-loaded lever that automatically advances the string -- you don't need to bump the head. When the string runs out, you can exchange the spool for a preloaded one, or you can simply rewind the string head.
Unplug the trimmer or remove the battery. Lay it on a flat surface so that the trimmer head is facing you.
Grasp the two tabs on the sides of the trimmer head cap with one hand and depress them. Pull the cap off and remove the spool.
Look inside the head, inspect the spring-loaded lever and replace it if it has come off its post. Clean grass and dirt from inside the head with a clean rag. Pull off any old string that remains on the spool and wipe grass and dirt off the spool with a rag.
Unwind about 2 feet of round 0.065-inch microfilament string from a bulk spool and make a 3/4-inch bend in the end. Feed the end through one of the anchoring slots on the rim of the spool and push the end into the hole adjacent to that slot.
Wind the string around the hub in the direction of the arrow displayed on the top of the spool. Lay each winding next to the previous one and avoid crossover. Stop winding when the outer edge of the string is flush with the recesses on the rim of the spool. Cut the string from the bulk spool with a knife.
Feed the end of the string through the hole in the string head and replace the spool onto the post inside the housing. Give the spool a little twist to seat it onto the automatic-feed lever.
Replace the cap by pushing it down onto the head until the two locking tabs click. Pull the end of the string out and cut off any part that extends past the trimmer deflector.