Wrap the chain around the large chainring and largest cassette cog (bypassing the derailleurs) with the flat edge facing away from the chainring and cassette. Add two inner links and two outer links where the chain starts to overlap. See details on the eTap AXS Systems Manual here.
You can also see chain sizing in the eTap AXS System Installation video here: https://youtu.be/Bx-kB7rNFJQ?t=379