One approach to back chaining is:

• Teacher does all but last part, learner does the last part etc. Points out each step. For example teaching a child to put on a jacket.
  • The teacher/parent does all but the last part of the zipper, child does the rest
  • Next time, the child does the full zipper
  • Next time, the child hooks the zipper together, then full zipper
  • Next time, the child does second arm, then hooks zipper, then full zipper
  • etc
• You would most likely practice at each step and few times before moving to the previous step.
• The tag point could be the last step or any step in the middle