Labels and cross-references

Never write "see Section 3" by hand

If you write a number by hand and later add a section, every number is wrong.
LaTeX solves this with labels. Tag something:

\section{Methods}\label{sec:methods}

The name sec:methods is just a nickname you choose.

$A label names an equation; ref prints its number and updates automatically$

Refer to it anywhere

Then point at it with \ref — LaTeX inserts the current number:

As shown in Section~\ref{sec:methods}, ...

The ~ is a space that never breaks across lines, so "Section" and its number stay together.

It needs two passes

On the first compile LaTeX learns the numbers; on the second it fills them in.
Our Run does this for you automatically. (The same trick powers a table of contents.)

Common mistakes

Put \label{...} after what you want to name, then refer to it with \ref{...}.
Compile twice so cross-references settle to the right numbers.

Label the Methods section with \label{sec:methods}, then refer to it from the last line with \ref{sec:methods}. LaTeX fills in the correct number for you.

\documentclass{article}
\begin{document}
\section{Introduction}
\section{Methods}
We followed a careful procedure.

As shown above, the method is simple.
\end{document}

Click Run to compile and preview the PDF.

The same numbering machinery builds a table of contents. Add \tableofcontents as the first line of the body — LaTeX lists every section automatically.

\documentclass{article}
\begin{document}
\section{Introduction}
\section{Methods}
\section{Results}
\end{document}