I realize that the best tip I have for anyone who wants to use a sling would be this: master the THREADING of the sling from the get go. The fabric has to be spread well on the rings to allow the user to adjust only a certain part by tugging the fabric on that side of the sling.
Here are the two videos to help you go through it step-by-step:
This shows where to tug if you want to adjust the outer rail/top rail or the inner rail/bottom rail. It also shows how to use the sling as a nursing cover.