Hi @paul_mendes
Well small but this is a broad topic - before starting this i hope you learned all the components of magento 2 and directories structure along with its coding standards !
Here i am sharing link , which does exact same functionality which you ask - https://magento.stackexchange.com/questions/200462/how-to-create-and-add-backend-form-to-custom-admi...
Gone though each step and create a custom module for the same !
It will works !
if issue solved,Click Kudos & Accept as Solution