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 I believe this is called a market buy order and I've looked at the steps on how to do this but I think they're outdated or wrong.
To buy one Bitcoin? You're still far from that, you need extra $6500 or so, based on the current price for one Bitcoin.
As for buying Bitcoin on Coinbase you will have to be fast about it, because if you delay and the price range changes, the purchase you're about to make will fail and you're going to have to restart your purchase.
But, a good suggestion is that you check out the selling orders on that trade page and then click on any of the sell orders and you will buy from them. If you keep making use of buy order you might not finish up before the price changes.