I connected from JTAG
pin 7 -> XIAO SWDIO (underneath the board)
pin 9 -> XIAO SWDCLK (underneath the board)
pin 15 -> XIAO RESET (underneath the board)
pin 4 -> XIAO GND
pin 1 -> XIAO +5v
My XIAO looks different from the bottom. It has SWDIO, SWDCLCK, RESET, GND on the left and VIN, GND on the right.