how to customize my headers?

Question

I want my part/section title to have underlined and be bald and increase its size. I can't find how to make the wanted setting stay as my default. How can I do this?

Answer 1

\documentclass{book}
\usepackage{sectsty}
\sectionfont{\underline\textbf\Large}
\usepackage{lipsum}
\begin{document}
    \chapter{This is a chapter}
    \section{This is your section}
    \lipsum[1]
\end{document}