This should be simple, but I can't seem to make the following happen:

I have a project plan for construction.  The majority of the tasks follow in a linear fasion (dig the hole, then pour the foundation, then installl window well, then backfill).  Those work fine.  But I need a set of deadlines for ordering material.

I want the deadline for ordering the window wells (for example) to be 5 days before backfill.

So whatever date backfill works out to be, I want the window well order deadline to be 5 days before.

How do I do this?