How would I set section numbering to a specific format - e. g. 1 (first level) -> 1. (second level) -> 1.1. (third level) in Lyx?

Question

I am creating a document for school and I am required to follow a specific format for numbering - that is: "1", then "1.", then "1.1." and so on. I am a Latex and Lyx beginner and haven't found a way to do this in this specific way (or in a simple enough way that I could follow). Should I use some package? Is there some preamble thing for this? Or would I have to do this manually?

Thank you in advance for your help.

Edit 1: Posting a MWE. I am using the article document class with manually changed section numbering to start from 0. Code:

\documentclass[english]{article}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\setcounter{secnumdepth}{4}
\setcounter{tocdepth}{4}
\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
\setcounter{section}{-1}
\makeatother
\usepackage{babel}
\begin{document}
\title{Title}
\author{John Doe}
\maketitle
\section{Preface}
\section{Characteristics}
\section{Division}
\subsection{Group A}
\subsubsection{A}
\subsubsection{B}
\paragraph{B1}
\section{Conclusion}
\end{document}

Screenshot from Lyx:

Answer 1

A brute-force solution is to run

\renewcommand\thesubsection{\arabic{subsection}.}
\renewcommand\thesubsubsection{\thesubsection\arabic{subsubsection}.}
\renewcommand\theparagraph{\thesubsubsection\arabic{paragraph}.}

Unfortunately, this will create unwanted issues if you ever need to cross-reference subsection-, subsubsection-, and paragraph-level units. I therefore suggest you take the following, indirect or "circumspect" approach. I picked up this approach up some years ago, while reading the book "The LaTeX Companion" (2nd ed.).

\documentclass[english]{article}
\usepackage[T1]{fontenc}
%\usepackage[latin9]{inputenc} %% Are you absolutely sure about 'latin9'?!
\usepackage{babel}
\setcounter{secnumdepth}{4} % default is '3'
\setcounter{tocdepth}{4}
%% Brute-force method:
%\renewcommand\thesubsection{\arabic{subsection}.}
%\renewcommand\thesubsubsection{\thesubsection\arabic{subsubsection}.}
%\renewcommand\theparagraph{\thesubsubsection\arabic{paragraph}.}
%% Circumspect method:
\makeatletter
% the next 4 lines are straight from "The LaTeX Companion", 2nd ed.
\def@seccntformat#1{@ifundefined{#1@cntformat}%
   {\csname the#1\endcsname\quad}   % default
   {\csname #1@cntformat\endcsname} % enable individual control
}
\renewcommand\thesubsection{\arabic{subsection}}
\newcommand{\subsection@cntformat}{\thesubsection.\quad}
\newcommand{\subsubsection@cntformat}{\thesubsubsection.\quad}
\newcommand{\paragraph@cntformat}{\theparagraph.\quad}
\makeatother
\begin{document}
\setcounter{section}{-1}
\section{Preface}
\section{Characteristics}
\section{Division}
\subsection{Group A}
\subsubsection{A} \label{sec:A}
\subsubsection{B} \label{sec:B}
\paragraph{B1} \label{sec:B1}
\section{Conclusion}
Cross-references to subsubsections \ref{sec:A} and \ref{sec:B} 
and to parapraph \ref{sec:B1}.
\end{document}