Please advise best and easiest way to travel from Mong Kok to Yuen Long Markets in Hong Kong

Both bus and MTR are good means to go.
By MTR: at Mong Kok, take Tsuen Wan line to Mei Foo, where transfer to West Rail line and get off at Yuen Long. Then walk to your destination.

By bus: walk to FIFE STREET MONG KOK stop to take bus no. 68X and get off at YOHO TOWN. After that, you can walk to Yuen Long market.