Multiple Award Schedule Desk Reference (2008)
Here is the official copy  from a GSA website and here is one with my comments included. This is the principal GSA publication on Schedules ordering.

NAICS Code to Service Schedule
Before you post something on FedBizOpps, use this NAICS Code cross-reference to point you to a service Schedule or SIN. If you decide to go the Schedule route, you won’t be doing anything with FedBizOpps. This cross-reference is also a good screening tool for service purchase requests: Is there a Schedule that might meet this type of requirement?

How Do I Find the Schedule Contract Clauses?
Ordering agencies need to know what clauses the Schedule contracts already contain so they do not conflict with or duplicate the contract when it comes to the clauses they decide to include in their orders.

Why Do Federal Agency Contracting Officers Use the Schedules?
Get behind the marketing slogans you see on some GSA web pages for the real contracting (meaning FAR-based) reasons Federal agencies use the Schedules rather than create their own IDIQ or do a non-Schedule procurement.

Keep It Simple (Not FAR Part 15)!
One of the Schedules ordering pitfalls common to some contracting activities, especially among 1102s with mainly FAR Part 15 experience who haven’t done much Part 13 simplified acquisitions (which closely resemble Schedule orders), is to make their Schedule orders look like overly complex Part 15 acquisitions.  The problem there isn’t just inefficiency but that courts and GAO have consistently held that if you make your Schedule order look like a Part 15 procurement, you will be held on review to those higher Part 15 procedural standards.  Nothing in FAR Part 15 applies to Schedule orders, so keep it simple.

GSA Order Checklist
GSA has developed an ordering checklist as part of the free online course “Using GSA Schedules – Customer”

Schedules Training Suggestions  - - Where to Find Out More About Schedule Ordering?
A great list of more online and other references to expand your knowledge of Schedules use.