So I own $1000 in bitcoin but I want to invest the remaining amount to accumulate a whole bitcoin. The prices fluctuates every second so how can I do this on coinbase? I believe this is called a market buy order and I've looked at the steps on how to do this but I think they're outdated or wrong.
Owning a whole number of bitcoins have never been an easy task at least in the current market situations. If you have $1000 but have the will to trade with whole number of bitcoins than you can move onto leverage trading. You can trade with 1 BTC on futures platforms just by holding some amount as collateral. $1000 would be a great collateral and you can easily trade above 1 BTC but there comes huge risk.
Your coins might be liquidated any time if the price you predicted went in opposite direction. This leads to huge loss and hence you should first try trading on various testnets so that you can test your skills and then move onto real trading if you find any profits in demo trading.