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.
Even though the host Linux PC recognizes the board (like as Onda (unverified) V972 tablet in flashing mode on lsusb), sudo sunxi-fel version may return ERROR: Allwinner USB FEL device not found!. In that case, you need to make a udev rule.
/etc/udev/rules.d/50-sunxi-fel.rules:
SUBSYSTEMS=="usb", ATTRS{idVendor}=="1f3a", ATTRS{idProduct}=="efe8", MODE:="0666"
If you want to build u-boot by yourself, python must be Python 2.x as of 2020-02-08. For example, you can't use the latest Fedora (F31) because the default interpreter of python is python 3.x.