I need to use the scalerel package available form CTAN. What do I need to do to use/install the package after I download and unzip the directory? I use TeXworks as my LaTeX editor on my Mac.
AT ALL costs avoid manual install If a package is available use your TeX manager to install it in sync with all the hundreds of other packages so as to maintain the interdependent relationships such as font mapping.
[EDIT since I posted this answer Moewe has produced a nice generic answer for all packages see How can I install a package on MikTeX? ]
If your TeX is MiKTeX then run MiKTeX-console go to packages select scalerel ask the manager to download and install. It should then do all the other necessary rebuilding database tasks. However it should do no harm to run TASKS update filenames database etc.
If your Tex is Tex Live run TLMgr --gui similarly locate scalerel ask the manager to download and install 
In this exceptionally simple case just unzip and keep all the files in the one folder DO NOT open scalerel.tex use yourOWN.tex file to load the adjoining scalerel.sty but read the scalerel.pdf first.
If you can not find a package easily in the tex manager they both allow you to filter search on just a part word so either would have listed scalerel using e.g. constrain
If you must download manually from Ctan first look for a-well-built.tds.zip anything else ask before download.
tlmgr install scalerelto install a package – user185220 Apr 9 '19 at 12:50