How do I substitute skim milk for dry milk in a muffin recipe?
You don't, it will not work.

You want to substitute liquid milk for dried milk. 

What you don't realise is that the dried milk in such a recipe is acting as a bulking agent, ie. the dried milk is already substituting for some flour.

If you don't have dried milk, you should look for another recipe that doesn't use it, if you want to be certain of reliable results.