How to create a .ist file for the purpose of customizing the index?

Question

I would like to create a .ist style file to change the layout of my Index. I am using book, hyperref and imakeidx. But I am at lost when it comes to creating the .ist file.

Basically, I would like to have the Index with bold alphabets before a new group of words begin. Something like this:

Index

A
angry
apple

B
ball
beer

My Codes:

\documentclass[11pt,a5paper,twoside]{extbook}
\usepackage{imakeidx}
\usepackage[colorlinks=true]{hyperref}
\makeindex[program=makeindex,columns=2,intoc=true]

\begin{document}  
\tableofcontents
\chapter{This is the First Chapter}  

She was angry \index{angry}  that she wasn't given an apple\index{apple}.  
He drank some beer \index{beer} and then played ball \index{ball}.  

\printindex

\end{document}

Answer 1

Create a pyro.ist file in the same directory as your main file with the following contents:

heading_prefix "\n\\noindent\\textbf{"
heading_suffix "}\\par\\nopagebreak\n"
headings_flag 1

and change the call to \makeindex to

\makeindex[program=makeindex,options=-s pyro,columns=2,intoc=true]

That's all. Your .ind file will look like

\begin{theindex}

\noindent\textbf{A}\par\nopagebreak

  \item angry, \hyperpage{3}
  \item apple, \hyperpage{3}

  \indexspace

\noindent\textbf{B}\par\nopagebreak

  \item ball, \hyperpage{3}
  \item beer, \hyperpage{3}

\end{theindex}