In general, there are two ways to reach hysterises
the first approach is using a one-stage comparator ,Allen's book give an example.
the second is you can use a one or two stage comparator followed by a smitter.
try to make the width-length ratio of M9 and m10 different.
two tips to form a hysteresis:
the first is positive feedback.
the second is that the circuits for two cases(vp>vn and vp<vn) are different.