% 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