First you make three sectors, the middle one being the lift sector. The left one has a lower floor height and the other higher. The lift sector has the height of the highest sector. Then double-click the linedef which needs a lower texture (joining lower sector and lift sector) After you double-clicked that linedef you use type 62 linedef with sector tag 0 and give it a wall texture. 62 is "Switch repeatable lower and raise platform". Next you double click the linedef which enters the lift from higher sector side. On that higher sector linedef you use 88 linedef type with sector 0 which means "Walk repeatable lower and raise platform" and give it the needed texture.