\documentclass{standalone}
\usepackage{tikz}
\usepackage{siltex}
\usetikzlibrary {arrows.meta,positioning}
\tikzset{nodes={text height=.7em, text width=2.8cm, align=center,
draw=black!50, thick, font=\footnotesize, fill=white},
>={Stealth[round,sep]}, rounded corners, semithick}

\begin{document}

\begin{tikzpicture}[on grid,y=1.2cm,x=3.2cm]
    \node (pd) {Problem Description};
    \node (sp)[below=1 of pd]  {Specifications};
    \node (id)[below=1 of sp]  {Initial Design};
    \node (im)[below=1 of id]  {Implementation};
    \node (te)[below=1 of im]  {Testing};
    \path[->] (pd) edge (sp)
              (sp) edge (id)
              (id) edge (im)
              (im) edge (te);
\end{tikzpicture}
\end{document}