It could only be done in one way.

They must kidnap her.

Sir Joseph was ordered to be ready to accompany his liege on this perilous enterprise in ten minutes.
"And mind," said Stroke, gravely, "we carry our lives in our hands." "In our hands!" gasped Sir Joseph, greatly puzzled, but he dared ask no more, and when the two set forth (leaving Agnes of Kingoldrum looking very uncomfortable), he was surprised to see that Stroke was carrying nothing.

Sir Joseph carried in his hand his red hanky, mysteriously knotted.
"Where is yours ?" he whispered.
"What meanest thou ?" Sir Joseph replied, "Oh, nothing," and thought it best to slip his handkerchief into his trouser-pocket, but the affair bothered him for long afterwards.
When they returned through the Den, there still seemed (to the unpiercing eye) to be but two of them; nevertheless, Stroke re-entered the Lair to announce to Agnes and the others that he had left Gavinia below in charge of Sir Joseph.

She was to walk the plank anon, but first she must be stripped that Agnes might don her garments.