Well there are a number of different forms of mines but I presume you're referring to pressure sensitive or release pressure anti-personnel mines. While the exact nature of it will vary from model to model generally a landmine will have some form of safety pin and/or a cap that is placed over the pressure sensor. The sapper will position the mine, conceal it if deemed necessary/possible and then remove the safety devices. When laying a minefield this will be done systematically while moving across the area with set lanes that are free of mines to allow friendly forces to move through the area where necessary with the pattern of mine distribution being recorded.