hoe does darcy assist Lydia in getting Wickham to marry her?

Mr. Wickham still was planning on making his fortune by marrying a woman with money. Therefore, Mr. Darcy had to, in fact, pay Wickham (and pay off his debts). He also helped him get a new commission in the regular army, which Wickham couldn't have done by himself.

By Mr. Darcy arranging for Wickham to marry Lydia, he preserved not only her reputation but also her sisters' reputations. He removed the disgrace from the Bennet name. Yes, Elizabeth thanked him for restoring Lydia's reputation, because Mr. Darcy not only had to use his own money to do so, but he had to reason and be often in the company with a man whom he - justly - detested.