To organise a car boot sale you have to first find out if you need permission from the loal authorities. If the boot sale is held within the curtilage of a building you would have to apply to the Local Planning Authority for planning permission. With respect of one off car boot sales such as many schools and churchs hold for charitable purposes, providing they are properly organised for instance using stewards for the direction of vehicles, liasing with neighbours and the police and no complaints are made to the Council it would not be necessary to submit a planning application. If the car boot sale was held in an open area not within the curtilage of a building for example in a field in the open countryside, under permitted development rights you can hold 14 such sales a year without the need to make a planning application. However, every site is unique so it would be advisable to check with the Local Planning Authority prior to holding a sale. Once again using stewards for the direction of vehicles and liasing with any neighbours and the police is advisable. After deciding on a location, begin to advertise for the events. you can use posters, flyers or advertisements in the newspapers.