The answer will have to be from the books as the show has no info in this regard as far as I know.
First thing first, There are varying accounts about his earlier performance. Tyrion says Littlefinger increased Gulltown's revenues threefolds and that It was the Crown's revenues which were increased tenfold. Lysa Arryn says he increased Gulltown's revenues tenfolds.
Ten years ago, Jon Arryn had given him a minor sinecure in customs,
where Lord Petyr had soon distinguished himself by bringing in three
times as much as any of the king's other collectors. King Robert had
been a prodigious spender. A man like Petyr Baelish, who had a gift
for rubbing two golden dragons together to breed a third, was
invaluable to his Hand. Littlefinger's rise had been arrow-swift.
Within three years of his coming to court, he was master of coin and a
member of the small council, and today the crown's revenues were ten
times what they had been under his beleaguered predecessor . . .
though the crown's debts had grown vast as well. A master juggler was
ACOK - Tyrion VI
My father said he was too lowborn, but I knew how high he’d rise.
Jon gave him the customs for Gulltown to please me, but when he increased the incomes tenfold my lord husband saw how clever he was
and gave him other appointments
ASOS - Sansa VI
I would say he used the same tricks he used when he was master of the coin, if that performance is built and perfected from his previous way of making the dragons breed.
He did not believe in putting gold in a vault and locking it up. He used to invest the revenues in ventures to generate more revenue. Furthermore, he was great at picking the right men for the right job, which enhanced efficiency of bureaucracy and therefore the revenues. He did not care for a man's birth as long as he was fit to do the job. And almost always, these men proved to be more qualified than their highborn counterparts.
Oh, he was clever. He did not simply collect the gold and lock it in
a treasure vault, no. He paid the king's debts in promises, and put
the king's gold to work. He bought wagons, shops, ships, houses. He
bought grain when it was plentiful and sold bread when it was scarce.
He bought wool from the north and linen from the south and lace from
Lys, stored it, moved it, dyed it, sold it. The golden dragons bred
and multiplied, and Littlefinger lent them out and brought them home
And in the process, he moved his own men into place. The Keepers of
the Keys were his, all four. The King's Counter and the King's Scales
were men he'd named. The officers in charge of all three mints.
Harbormasters, tax farmers, customs sergeants, wool factors, toll
collectors, pursers, wine factors; nine of every ten belonged to
Littlefinger. They were men of middling birth, by and large;
merchants' sons, lesser lordlings, sometimes even foreigners, but
judging from their results, far more able than their highborn
ACOK - Tyrion VI