I assume you know where to put this :
$game_temp.fade_type = fade
$game_player.reserve_transfer(map_id, x, y, direction)
# Fade/Fade Style = [0; Default, Black], [1] White, [2] None
# For direction: [0; Default, Retain], [2] Down, [4] Left, [8] Up, [6] Right
Copied from rpgmakerweb