Svaret måste vara från böckerna eftersom showen inte har någon information i detta avseende så vitt jag vet.

Först första, det finns olika konton om hans tidigare prestanda. Tyrion säger att Littlefinger ökade Gulltowns intäkter trefaldiga och att det var kronans intäkter som ökades tiofaldigt. Lysa Arryn säger att han ökade Gulltowns intäkter tiofaldiga.

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 Petyr Baelish.
ACOK - Tyrion VI

och

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

Jag skulle säga att han använde samma knep som han använde när han var mästare på myntet, om den prestationen är byggd och perfektionad från sin tidigare sätt att drakarna odlar.

Han trodde inte på att lägga guld i ett valv och låsa upp det. Han brukade investera intäkterna i ventures för att generera mer intäkter. Dessutom var han bra att välja rätt män till rätt jobb, vilket förbättrade effektiviteten av byråkratin och därmed intäkterna. Han bryr sig inte om en mans födsel så länge han var lämplig att göra jobbet. Och nästan alltid visade sig dessa män vara mer kvalificerade än deras högfödda motsvarigheter.

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 with hatchlings.

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 predecessors.
ACOK - Tyrion VI

Vi vet inte

Böckerna har bara följande avsnitt om hur han gjorde det och att Petyr alltid verkar i bakgrunden är förnuftig att anta att hans metoder inte är kända.

Petyr's breath is always fresh . . . he was the first man I ever kissed, you know. 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, even brought him to King's Landing to be master of coin.
A Storm of Swords, Sansa VI

Det är också värt att notera följande avsnitt om Arryns från Gulltown som är uppmärksam på att vara rik från att gifta sig med köpmän.

There are several branches of House Arryn scattered across the Vale, all as proud as they are penurious, save for the Gulltown Arryns, who had the rare good sense to marry merchants. They're rich, but less than couth, so no one talks about them.
A Feast for Crows, Alayne II