When you are ready to create an offer, create a Sales Proposal in the Sales Lead's feed.
A Sales Proposal is a special activity for tracking offers and orders. Set the sent date when you have sent your offer. The sum fields should correspond to the estimated value of the sale at this point.
When you've created the offer, the related Sales Lead is handled for now and you can mark it as finished. The Sales Proposal is not finished after sending it, though. Move it to whenever you plan to follow up on your offer. This can be the next day or week depending what you are selling.