So I own $1000 in bitcoin but I want to invest the remaining amount to accumulate a whole bitcoin. The prices fluctuates every second so how can I do this on coinbase? I believe this is called a market buy order and I've looked at the steps on how to do this but I think they're outdated or wrong.
Find a good bitcoin wallet where you could store your bitcoin most of the time the bitcoin wallet you use already have a market or an exchanger that you could buy bitcoin.
Like for example coinbase.com just create an account in the websites or in the mobile application and then convert your money to buy bitcoin directly in the application,
you could easily use your card to have a direct payment in coinbase to buy bitcoin in the website it will be a huge price since 1 bitcoin was a 7k$ in the market.