single balanced mixer

I don't understand how does single balanced mixer cancel RF to IF feed through , and also how does double balanced mixer cancel LO to IF feed through ?

A balanced mixer can be adjusted to get LO/RF leakage better than 20dB but only over a narrow band. LO/IF leakage is usually high, so low-pass filter must be used.
For LO/RF leakage it is also good to use filters.

The "how" question can only be answered if you do make a balanced mixer. Balancing means selecting diode pairs plus adjusting parasitic reactances by tuning.
Diode pairs in mixers function as switches, so a good LO input power is needed for it. For a lower LO input the diode pair cannot be balanced.

Check e.g Mini-circuits mixer designs and parameters.