First thing to do is recognize the eye on the left. If Black a, White b, etc. So forget that, it's done. On the top, how can Black get a second eye? If Black c, White d, Black can extend starting with e until he can capture three, resulting in a second eye. Black f makes it even easier. White needs to find a way to prevent the connection at c or f, or at least turn it into a false eye.
Simply blocking doesn't do it.
is a good start, but
jumps in too soon, creating essentially the same situation as before. Black can easily make eyes.
threatens
, so
to block.
puts the Black group in atari, so Black cannot block at a.
If then
. The captured white stone is only a false eye. Ta-da.
An altnerative to . This time,
threatens
.
at
, a throw-in to keep the eye false.