The base FHIR specification describes a set of base resources, frameworks and APIs that are used in many different contexts in healthcare. However there is wide variability between jurisdictions and across the healthcare ecosystem around practices, requirements, regulations, education and what actions are feasible and/or beneficial.
An Implementation Guide is a collection of Profiles, that make conformance rules (technically, CapabilityStatement, StructureDefinition, ValueSet, etc.) that, together with textual guidance, provides instruction on how to make use of FHIR in a particular problem space.
There is no prescribed way of developing an Implementation Guides as long as it contains the elements described by the Standard. Over time however, two approaches have surfaced as preferred authoring methods. One uses the the IG Build tool developed by HL7 International, the second and more recent, is part of the Furore FHIR Registry tool called Simplifier.net.
For further details please refer to the relevant pages: