This can't be a complete tutorial, sorry

Get Audacity from:  https://www.audacityteam.org/

There's documentation starting here:  https://www.audacityteam.org/help/documentation/

Here's a tutorial someone else has done: Warning - Loud!

Here's what I actually do

Optional steps you don't need but I sometimes do:

• clean up noise / background hum
• Amplify the audio if it was recorded a bit quiet
• clip silent or uninteresting portions off the start and end

And here's the method and settings I actually use for voice clips to convert to MP3:

It's served me well since March 2020!