When you apply online, you enter all your referees’ details in the relevant sections. The required details are the same for both the first and second referee. You must obtain all the personal details from your referees before you start filling in the form.

See: Who can be a referee.

It might be prudent to check with your referees that they are fine to give all these details to you first.
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Once you complete the screen above you get the following one:

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The form does require you to enter a passport number if the referee has one. They can still be referees without a passport as the form asks whether they have one, however, the Home Office would be aware of whether people have a passport or not.

They can still be referees even if their passports have expired.

You can give either their work or home address. If they have been at their current address less than 3 years, they should also provide their previous address(es).

Referee declarations and photos

Once you have submitted the form, you will be presented with a page where you can download your referee declarations. These forms should be printed and signed by your referees after you have affixed your photo to the page.

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The photo that goes on the referee declaration is the applicant's photo, not the referee's photo.

There's no need to write anything on the back of the photo before gluing it to the form, this was necessary when the documents were sent as hard copies by post, in case the photo became unstuck, but it's not necessary if the completed form is scanned and uploaded in digital format.

The forms can be signed and dated either before (as above) or after the form is submitted online. You will find direct links to them below.

You cannot use pages from the old paper forms. You need to download the declarations and get them to sign them.
The completed referee declarations have to be scanned and uploaded with the rest of supporting documents, to the UKVCAS screen, unless your appointment includes scanning services.

See: Uploading documents to the UKVCAS interface - UKCEN Citizenship and Residence for European Nationals and their families

Direct links to referee declarations to be used with the online form: Please note that both forms are identical, each referee has to sign and date one of them, that's probably why two are provided.