Knowing their estimated costs and profit requirements the arbitrageur determines a minimum difference they need between the futures’ prices and the spot price before they will enter the market. They then monitor the price difference between Bitcoin futures and the Bitcoin exchanges and if large enough they act to profit on that gap. For example, if a specific Bitcoin future (e.g., February contract) is trading sufficiently higher than the current Bitcoin exchange price they will short that Bitcoin future and hedge their position by buying Bitcoins on the exchange. At that point, if they have achieved trade prices within their targets, they have locked in a guaranteed profit. They will hold those positions until contract expiration (or until they can cover their short futures and sell Bitcoins at a profit).
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There's a lot of ins and outs which can get confusing in arbitrage trading. We won't go into the technicals of why futures contracts trade at a premium to spot price. You can read a full explanation here. If it's not of interest to you, all you need to know is that there's a tendency, the further out in time the futures contract expires, for the premium to spot to be higher and higher in nominal percentage terms.