This is the what I have gathered. Page was an FBI asset for years.
How do you get to Tap Trump?
You get a FISA warrant ON YOUR OWN AGENT on the basis he was in contact with Russians Criminals. (True = he was while building an FBI case - leave this bit out)
Then send him along to advise the Trump campaign.
Presto you now have enough of a relationship to someone under the top FISA surveillance.