For more automation, you may want to define a new command \bitem
, as follows:

\documentclass{article}
\newcommand\bitem[1]{\item{\bfseries #1}\\}
\begin{document}
\begin{enumerate}
\bitem {Specify a mean equation for the returns}
by testing for serial dependence of $r_t$ an, if necessary, building an ARMA model
\bitem {Test for ARCH Effects} using the residuals of the mean equation
\bitem {Specify a volatility model} (if necessary)
\bitem {Estimate the complete model}
via joint estimation of mean an volatility parameters
\bitem {Check the fitted model} and refine if necessary.
\end{enumerate}
\end{document}