You will only get jobs with companies with whom you are signed up.
Most of the times you will either select or request the job from the company's website job board (and usually you need to sign in as a shopper to see that job board).
Once you are accepted for the job you can open up the instructions that are more specific than the general overview of the job you saw prior to requesting/accepting the job.
The instructions will specify how and when the job is to be reported. 95% of the time you will be submitting your report on line (and most often you can see a copy of the report before you ever go do the job so you are sure what questions you must be able to answer).
The instructions will also specify what 'proof of visit' you are to submit and how you are to submit it. The more 'old fashioned' companies may still have you mailing in receipts, while some may have you email a scan of the 'proof' and most will have you upload it with or at the immediate end of your report.
I have only one company left that wants me to submit reports via email or fax. There are a couple of companies that require or will accept a phone report. But by far the most frequent submission is directly to an on-line form.