It would be better to say "I have gone through the details below".
:up: And even if you want to use "below" as an adjective coming before "details" - a piece of rather dated official jargon - "details" still need their "the".
(Also, as a writer, you need to consider whether the reader
cares exactly where the details are; is it enough to say 'I've been through these details' or something of the kind - perhaps as little as 'I've had a look at this'? I'm not saying it always
is enough, but you should always think about adding extra stuff for your reader to interpret.)
b