Commission jobs, scope it first. Submit a written proposal for the client, incorporating any written material they have submitted to you.

At this point it is a process of clarification and reiterative. NO artwork, not even a preliminary sketch is ever submitted for the project. Your existing portfolio stands for the quality of your work, and your references from past clients.

If the "client" baulks at this point, then you know you have trouble. Politely thank them for contacting you, and wish them well for their project. End of story.

Should the clarification of proposed project reach a point of mutual understanding, then and only then do you do a project costing, revise it, ensuring a profit margin. A starting point is, you have your hourly rate. Take your estimated hours for the project and multiply by 2.5. That is a starting point for your costing proposal.

Now, before you submit that, you also split the costing up, with a nonrefundable deposit, 30-50%, and then milestone payments. Also additional to this costing proposal, are the terms and conditions for the number of variations, and the number of revisions for a chosen variation. Anything outside of that is then charged extra. You state in your terms what these are.

So then bundle all of that together and send it back to your potential client. You will soon find out if you have some one serious, or a dreamer.

I learnt all of this the hard way myself.