Prepare USB: format it as FAT32 and copy your files
Insert USB into your Surface
Hold Vol-Down
Press PWR
Wait until the Surface logo shows and wait some more seconds
Release Vol-Down
To boot unsigned non Microsoft UEFI Apps you need to apply Yahallo first
Connect your Tegra3/4 via an USB A-to-A cable to an USB 3.0 Port of your PC
Hold Vol-Up
Press PWR
Release Vol-Up
it should immediately boot into APX mode
Use your payload sender to send data (Fusée Gelée)
Newer PCs can use an USB Type C cable instead of USB A-to-A cable