I am part way through a pair of mittens and realize that I will not have enough thread to finish. I plan to insert a different colour and then go back to my original colour when decreasing the tips. Can someone please walk me through how to switch from one colour to another an back? Do I cut and tie the yarn to the new colour?
If it is going to be several rounds before you use the color again, yes cut the yarn leaving a nice tail. You don’t really need to do anything special to begin the new color, just leave a tail of the new color and start knitting with it. Switch this process when you change back. When you are all done with your mitten, turn it inside out and hide in the tails closing any little gaps anywhere with the tails. You can turn back to the right side to see how it looks, if it looks good 
if not you can try again.