LearnLaTeX has some high quality tutorials,
resources, and more interestingly, a nice online compiler!

ℹ️ Also see my leaf.sty package for useful configurations.

General

LaTeX via Images

This is the last thing one should use since images aren't accessible, and terrible to manipulate. Many platforms, however, do not leave a choice (e.g. Github Markdown). As a stop-gap, use CodeCogs Equation Editor^{a} to generate SVGs of equations. Here's the Bayes' rule,

A good place is in the preamble right before any other package imports. In this
case, any subsequent import of the graphicx package can be done as

\usepackage{graphicx}

which will implicitly also pass the desired option. This is helpful when another
package wants to load graphicx with different options. Effectively, this
command will append to the options list. This may not always be the best solution
but should work for most cases.

Localize Settings

Just wrap everything inside

\begingroup
...
\endgroup

or

\bgroup
...
\egroup

Wrapping Figure Around Text

% "r" for right, "l" for left\begin{wrapfigure}{r}{0.5\textwidth}
...
\end{wrapfigure}

This is often helpful in two-column formats where the inline math can split over
multiple lines. To the reader, that looks ugly and distracting. The solution is
to simply wrap the equation with curly braces {}.

This inline equation ${a^2 + b^2 = c^2}$ will not split over multiple lines.

Split Long Equations

Inside align environments, use split. {}& for alignment and the usual new line \\ for split points.

\begin{align}
a = b + c \\\begin{split}
z^2 = {}&x^2 + \\{}&y^2
\end{split} \\
...
\end{align}

Tables

Fit Oversized Table

To make a table fit automatically across the width, use