Rule number one:  Use a stainless steel tube.  Those plastic tubes should be shoved somewhere.  Stainless slides in so much easier.

Next, with calf's head between your knees, insert the tube on the RIGHT side of the mouth, and point it towards the LEFT side of the calf.  Rub the roof of the calf's mouth to stimulate swallowing, and keep rubbing as you slowly push the tube in.  Only the most premature calves will still sometimes have a problem.  Other then those, tubes go in with no problems.