Once I have replaced a broken tightener on my 94 explorer, what is the best way to get the belt back on?

Hopefully you have a belt routing diagram right there in front of you as you open the hood. If you don't have a diagram you'll probably never figure out how to route the belt. Assuming you have a diagram route the belt around everywhere leaving the tightener until last, the tightener should be spring loaded, use a wrench or socket and handle to releive the tension on the tightener and route the belt under the tightener.

I should add one more thing. After the belt is completely installed, check each pulley to make sure the belt is not a rib off, ie sticking over the side of the pulley. I've seen this happen a lot.

It may take three hands to do it. Two to hold the tensioner 'up' or 'back', another to fit the serp belt on.

http://www.autopart.com/tools/toolsmain/tool/T_A875.htm Also search 2700
These are pictures of a tool of the type normally used.
Auto Zone, Advanced Auto and O'Riley's have a tool rental or loaner program.