How did Macbeth and Lady Macbeth cover their treachery in Macbeth?

Let us remember that they didn't actually do an incredibly
good job of it. The first crime, that of regicide or the killing of Duncan, was actually
only possible to cover up through the killing by Macbeth of the two grooms that he and
his wife had framed for the murder by smearing them with Duncan's blood. This of course,
if we examine Act II scene 3 very carefully, makes Macduff very suspicious, as his
question of "Wherefore did you so?" makes absolutely clear when he asks why it was
Macbeth killed the grooms. Of course, it is just chance that the two sons of Duncan
decide to flee, giving Macbeth the perfect opportunity to blame them conveniently, as
they are not present to defend themselves.

If we then think
about the next crime, the killing of Banquo, the inconvenient appearing of his ghost
during the banquet scene of Act III scene 4 really threatens Macbeth's secrecy, and
clearly makes his lords very suspicious about his sanity, but also his involvement in
the crimes. If we look carefully at Act III scene 6, Lennox makes it clear that he is
very suspicious of the "convenient" way in which things have fallen in place for
Macbeth:

The
gracious Duncan

Was pitied of Macbeth:--marry, he was
dead:--

And the right-valiant Banquo walk'd too
late;

Whom, you may say (if't please you) Fleance
kill'd,

For Fleance fled. Men must not walk too
late.

The sarcasm and
suspicion in his voice is evident. Of course, after this, Macbeth moves from trying to
commit crime and treachery in secret to the open slaughter of Macduff's family, ending
any pretense of goodness whatsoever. So, all in all, we could argue that the efforts
that Lady Macbeth and Macbeth made to cover up their treachery were not actually that
effective, as Macbeth's suspicious killing of the henchman does not go unnoticed, and
the ghost of Banquo reduces him to near-madness.