tested on openbsd 7.8

install openbsd on bare metal

prepare your computer by backing up all data.

disable secure boot and enable uefi boot in the bios.
see how to do this on a thinkpad: x1 or t14.

prepare an usb drive with openbsd installer.
for example, on macos.

boot the installer.

choose (I)nstall and answer questions. for example:

Do you want the X Window System to be started by xenodm(1)? = yes
Which disk is the root disk? = sd2
Use (W)hole disk MBR, whole disk (G)PT or (E)dit? = G
Location of sets? = disk
Is the disk partition already mounted? = no
Which disk contains the install media? = sd1
Continue without verification? = yes

unplug usb drive with the installer and boot openbsd from the target drive.