If you want to visit in December then it would be best time to visit sikkam. you must travel through bike it would be better for you anyhow there will be too much cold in night time but you will get pleasent day. you should take bike from new jalpaiguri ( silliguri) and travel to sikkam. there are many agencies in silliguri which gives bike on rent for a day to a week. it will be better for you and you can enjoy too much on this trip also.