A microkernel implements a minimal number of abstractions that facilitate the realization of operating system services.
Mach's concepts are documented here.
Read OSF's Kernel Principles. Find it under the documentation link.
Mach's API is documented in OSF's Kernel API book. Find it under the documentation link. Skim it to get an idea of how the API looks and then try the following exercises:
Send messages using Mach's IPC mechanism (detailed description).
Implement your own pager. Write a server that synthesizes content on the fly and have a client map the object into its address space and print out the file.