How to long line- Search the internet, find a good book or ask an instructor.
Why do you long line-when used correctly to teach a young horse rein aids, to exercise a childs pony, to bring a horse back into work, to see how straight your horse moves from the ground, train lateral movements, build muscle and increase suppleness etc its very beneficial.
What do you need-You could use a bridle and bit or a headcollar, a saddle or a roller and long reins.
Make sure your horse is used to long reins first because many horses think that they have giant snakes draped around their hocks, Do it in a enclosed area for a start and remove all hazards (Poles and jump wings) etc. If you use a bit be careful not to be too heavy handed and as soon as they respond release the pressure on the reins. have a helper if possible stay at the horse's head for a couple of times until you feel confident.
Hope I've been Helpful x
You can use boots if you want to and also if you use a saddle use a spare stirrup leather to join the stirrups together under the belly to stop them moving, but twist the stirrup leather under the belly.