I did 7 years of writing InstallScript installers before ever picking up MSI. While there is a huge difference between procedural script-driven imperative installs and data driven declarative installs, they both do the same fundemental thing: deploy software.
I became an MSI Expert but studying everything I could on the domain, writing LOTS of installs and by blogging for 7 years and answering over 4,400 posts on the InstallShield community forums. The only way to go in my book is to have been there and done that.
So the first step in your quest should be to understand the Windows Platform and related technologies very thoroughly. These evolve over the years but you should get a decent understanding of:
Fundamentals
- Registry
- FileSystem
- NTFS
- ACL's
- DLL Types ( Win32, COM, .NET Assembly)
- Win32 API
- .NET Base Class Libraries
- Service Control Manager Drivers ODBC
- SQL IIS Active Directory ( GPO, LDAPand so on )
- Global Assembly Cache
- WinSxS Cache
- DLL Hell
- Good and Bad Installer Behavior
The second step is
Tools
Now let's start to writing installs. As Leslie ( Easter I assume ) said in another answer, pick a tool and learn how to use it to accomplish the above things. But don't stop there, as soon as you can go to the next step.
MSI
Start digging deep down into how your tool is working behind the scenes as soon as you can. Just as you can write C# in .NET and look at the IL with ILDASM, learn to use ORCA and see what is happening. Read the MSI SDK. Yes, it's rough and cryptic but I spent 3 months commuting beween DC and TX and I spent at least 16 hours a week traveling away from internet connections but nothing except the SDK to read. Read it, know it, live it... the cryptic help topics will eventually start to click and become second nature.
And finally, read my blog: DeploymentEngineering.com and every other blog you can find.