The long jump achievement consists of jumping 300 feet. All you need to do is make a portal on a wall and then another on the floor. Jump down through the floor portal. Don't just walk into the floor portal; jump in. Then, after you fall out of the wall portal, shoot a floor portal and look down at it to make yourself continue to fall without landing. The game will consider you to be adding distance to your long jump. After you think that you have jumped far enough, move so that you land on the ground instead of falling into the portal. If your jump was long enough, the achievement is automatically unlocked after you land on the ground. Otherwise, just try again and wait for a longer time before you come out of the jump.