As Mamyalynne has sort of indicated, it's simply a matter of filling a form in and submtting it to the court. Unfortunately the link to the form (in the page provided by Mamyalynne) seems to be dead but your daughter can download the form from here
http://www.familylaw.co.uk/system/uploads/attachments/0002/0821/D8.pdf
and the supporting notes from here:
http://hmctsformfinder.justice.gov.uk/courtfinder/forms/d008-notes-eng.pdf
Your daughter's local CAB could probably help her fill in the form (free of charge) if she's not sure about anything.
The relevant link from Mamyalynne's page provides information about assistance with the court fee if your daughter is on a low income.
Her husband could contest the statements submitted by your daughter (e.g. "I didn't commit adultery" or "We've not been separated for as long as she says") but, if the necessary circumstances for a legal separation order to be made have come about (and the court accepts that fact), the order will be granted.