Your country sounds a bit like Brunei. They have their currency tied to Singaporean dollar, which has much larger economy.

I did a quick check.

wp:Brunei dollar

(my bold)

Under the Currency Interchangeability Agreement in 1967, the Brunei dollar is interchangeable with the Singapore dollar at par. As such, the Brunei dollar is accepted in Singapore as "customary tender"; likewise, the Singapore dollar is accepted in Brunei.[4]

"customary tender"—I like—I might put that expression in my story. Thanks. 🙂

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It seems Brunei has had a number of currencies in the past 100 or so years:

wp:Malaya and British Borneo dollar

1953 to 1967

wp:Malayan dollar

1939 to 1953

wp:Straits dollar

1898 to 1939

In 1931, the sterling was taken off the gold standard, putting the Straits dollar on a sterling-exchange standard.[2]

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That said, assuming you mean “how to keep my currency exchange rate from imploding”, you need to force somebody to pay the state something in that currency.

Again, I'm thinking of making it legal tender gradually.

For example, when a resident (citizen or otherwise) might have in, say 1938, paid wt:thons taxes, thon had to pay ≥60% of it in other foreign hard currencies, but thons sibling who worked for the government, might have been paid ≤50% in foreign currencies, and ≥50% in the country's currency. In, say, 1958, it might have been ≥40% of it in other foreign hard currencies, but thons sibling, might have been paid ≤30% in foreign currencies, and ≥70% in the country's currency. In, say, 1988, it might have been the government worker, now retired, gets 100% of thons pension in the country's currency, and in 1998, all taxes were paid in the country's currency, up to the present time.

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Most commonly that’s taxes, but if there’s some important export you can put a requirement that payments need to be done in your currency. Most of the time international currencies are fine for that because you can exchange them and prop up your currency this way

Which I suppose such is because Brunei allows people to bring in any amount of Brunei dollars that they traded for outside the country, or perhaps traded inside the country, unlike the former USSR which IIUC, was illegal, or essentially illegal, to bring in Soviet rubles, but rather one had to exchange them officially at official rates.

The thing about my fictitious country, they aren't a petrol-state or are similarly rich in resources, or at least a particular one, nor did they trade much. It's a bit mountainous—hilly at least—and essentially landlocked with no major port. Pretty well anything they could sell could be bought more cheaply elsewhere.

Currently I have it starting the 20th century as mostly agrarian (and perhaps mostly subsistence), pastoral, and some limited extraction. Attempts were made at industrialization mid-century, with mixed results. The last 30 years it was the tertiary sector that grew and did much to make it a prosperous country (e.g. areas of legalized gambling and prostitution; penalties for illegal drug use not much more severe than Western countries at the time; and a bit of a tax haven (presumably smaller countries are easier to manage economically than larger ones)).