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 I believe this is called a market buy order and I've looked at the steps on how to do this but I think they're outdated or wrong.
Your thoughts seem to be moving towards trading and I highly recommend not trying. Because when trading, you have to pay a fee when you order. Besides, if you are lucky, you will gain a few units, but if unlucky, you will lose a few units and lose extra fees. To be able to trade, you need to learn a lot from trading books.