First, the attorney must perform a conflict of interest check on you and your invention. If the attorney has a conflict, then he cannot legally write your application. You just wasted several hundred dollars and valuable time. Also, be prepared to pay additional hidden costs too, like fees for hiring a professional draftsman and filing fees. These additional costs can easily add several thousand dollars to the attorney's fees.
Attorneys generally require some money up-front.
So bring your checkbook and VISA to the law office. Once the attorney clears your credit, your patent application will be put on his docket to be written some time in the future. Patent attorneys can easily take from several weeks to several months to write an application. So be prepared for a long wait. Once the application is finished, you will have to pay the balance of all the legal fees.