此开卷第一回也.作者自云:因曾历过一番梦幻之后,故将真事隐去,而借"通灵" 之说,撰此<<石头记>>一书也

My try:

此开卷第一回也.
Hereby starts the first chapter (but why the 也??)

作者自云:
The author (作者) himself (自) says/reports (云) .

因曾历过一番梦幻之后,故将真事隐去,而借"通灵" 之说,撰此<<石头记>>一书也

The novel "The stone recordings" was written due to the occurance of a dream that tells a story that hides real events and that tells about meeting the afterlife/world of the dead.

Components:

Because (因)
a former (曾) experienced (历过) dream / illusion (梦幻;  番 as a measure word).

故将真事隐去
the story conceals (隐去 passive with the 将) a true event (真事)

and reports how thereby/there (而借) a connection to the afterlife was established (通灵)

The book "The stone memories/recordings" was written

也 to finish the 将-construction

Any other thoughts on that?