% using psfrag example eps
% /usr/local/texlive/2019/texmf-dist/doc/latex/psfrag/example.eps

\input psfrag

First without psfrag:

        \includegraphics[width=3.5in]{example.eps}

and now with:

        \psfragscanon
        \psfrag{p1}[][l]{$\ast$}
        \psfrag{p2}[][l]{$\ast$}
        \psfrag{p3}{$\cos(t)$}
        \includegraphics[width=3.5in]{example.eps}


\bye