Convert a Sales Order partially into an Invoice

SUGGESTED

We invoice our clients per milestone. So I would like to crate an order at the beginning of the project with the different milestones and every time a milestone is met, then issue an invoice corresponding to that milestone. Is it possible to convert partially the order into an invoice with only one milestone? I read that using Proposals is a good idea, but I do not know how to enable Proposals in my Sage. It does not appear anywhere. I am using Sage 50 Cloud.