If SPI flash isn't on the board, just remove the TF(microSD) card from the board and supply the power. It's the same if SPI flash is empty,
If your board have a SPI flash with preloaded image, pull D4(SPI CLK
) down to GND. You can use the metal part of USB micro B or TF card slot as GND. Diagram is in the comment below. Thanks: @ernestp for the correction and providing a diagram.
Or you can use the special boot image [link]. Just write it to a TF card and boot.
Don't forget to remove the GND connection after entering FEL mode otherwise you will never be able to write the boot image to the SPI flash.