Installing OpenBSD -current snapshots

I have successfully installed OpenBSD 6.6 release and would like to give
the Current Snapshots a try. I went to a mirror, and to:

Index of /pub/OpenBSD/snapshots/amd64/

I saw install66.fs (probably for usb memstick) and install66.iso (surely
for a cd/dvd) at ~450Mb. I picked the install66.fs, wrote it to a usb
thumbdrive, and it starts the install. When i get into the install it asks
where are the file sets? Humm, maybe it gets these online and it tries to
do this but no luck. It was late last night, and I checked to see if it had
written anything to my disk, which it had not, and went to bed. This
evening I'm looking a bit deeper at the snapshot directory and I suspect I
need to provide the install with base66.tzg at ~239Mb.

My question now is after downloading the base, do I need to un-tar it, and
how to I provide it to the install? I wrote the install66.fs to the usb
with the dd command. Not clear to me how to either manually copy the base
file set to the usb, or maybe leave it on an accessible directory on my
machine. Any help would be appreciated.

Clay Daniels