When you buy/sell via an exchange, try to use limit orders (try not to use market orders). On some exchanges, like GDAX, limit orders have lower fees than market orders. On GDAX, limit orders are free as long as they don’t fill immediately. Meanwhile, market orders result in a .3% fee, which is better than the 1.4% that Coinbase charges but not as good as 0%, especially if you are day trading. If your exchange rewards you for using certain order types, aim to use them.
Let’s say on January 2, 2017, you owned $1,000 and exchanged it into euros (EUR) with another market trader at a rate of EUR€0.9565, leaving you with EUR€956.50. Then, on November 24, 2017, the value of the US dollar had fallen from EUR€0.9565 to EUR€0.8380, so you decide to exchange your euros back into US dollars. After finding someone willing to sell their US dollars for your euros, you carry out the exchange at the new price. Your EUR€956.50 is now worth $1,141.40, leaving you with a profit of a little more than 14%.
No, the successful trader is not me. I’ve gotten lucky a few times and I’m still refining and trying out strategies; on the other hand, I’m part of communities of people who trade on a daily basis to grow their portfolios, and while some of the results can be attributed to luck, a majority of it is based on fundamentals, good habits, and experience.