You're not limited to taking the prices that are already up in the market. If you're not happy with the current market prices then you can also enter a prediction at your own price and wait for another trader to match that prediction on your terms.
You can set your own price to move to the front of the price line. If your price to buy shares is higher than any of the other buy orders waiting to be matched then sellers will see your order first. If you're selling shares and you set a price lower than any of the other sell orders waiting to be matched then buyers will see your order first.
You can also set your price for further down the price line and wait for the market to move in your direction. For example, if you wanted to buy shares but think they're a little overpriced you could set your price lower than the current highest market price. The market may not move in your direction and your order may not get matched, but you also won't buy shares at a price you consider too high.
To set your own price you simply enter that price when making your prediction. Then review and confirm your prediction and send it to the market to be matched.