A certificate can be generated with openssl. If you have a local Apache running, chances are high, you already have it somewhere.

I used the very same tutorial some months ago and it worked fine.

Just do this step by step. No need to really understand anything of this for now. In the end you will have your own certificate, and if you setup your webserver to use it, wou'll get a warning because it's not registered. But this will be fine for your use.