Basic function question

[Urs Liska]

Hello list members,

after fiddling around too long and getting lost with the basics, I 
decided to ask my question here.
I know it's basic, and I wonder why I get stuck this way, but obviously 
I'm confused and unable to find the right place in the docs.

I want to write a function "instr" to produce formatted markup that I 
can use like

{ c d e f^\instr "Vl." }

which should be translated to (e.g.)

{ c d e f^\markup \bold \italic \huge { "Vl." } }

Unfortunately I seem absolutely unable to find out how to define the 
function so I can attach it to the note like in the example.

Hopefully someone can either give me a working example or point me to 
the exact location in the docs where I can fully understand this issue.

Thanks in advance
Urs