(setq TeX-PDF-mode t) ;; use pdflatex instead of latex
(add-hook 'LaTeX-mode-hook 'flyspell-mode)
(add-hook 'LaTeX-mode-hook 'flyspell-buffer)

documentclass{minimal}
usepackage{tikz}
usepackage{forest}
begin{document}
begin{forest}
  [[]
end{forest}
end{document}