You should be able to find more detailed guidance on specimen preparation online, e.g.
http://extreme-macro.co.uk/mounting-insects/
https://extension.entm.purdue.edu/401Book/default.php
Or this older forum thread: https://forum.inaturalist.org/t/getting-started-with-insect-collecting/25396/4
Depending on the size of your beetles and the difficulty of identification, a microscope may not be essential, but for anything under about 1 cm you will probably find it easier to see what you are doing if you have some kind of magnification, whether a magnifier/loupe or a microscope.
At the risk of stating the obvious, the type of microscope that you want for this is a stereo microscope (sometimes called a dissecting microscope). I point this out because it took me rather longer than I would like to admit to grasp this basic difference in microscope construction. A stereo microscope has two eyepiece lenses* at slightly different angles to produce a greater depth of field/3D-effect which is helpful when working with insect specimens. A compound microscope has a flat field of view and allows for higher magnification; it is therefore good for looking at slide-mounted samples such as cells and spores and microorganisms but less suitable for preparing specimens because of the short working distance.
*Hence, if you speak a European language other than English, you might know this as a “binocular”, which can be somewhat confusing to those of us for whom binoculars are something used for looking at distant objects like birds.