Beat the best buying price - in brief

The procedure

Step 1. (more detail)	Select the ORDER BOARD by pressing the BUY/SELL menu button.
Step 2. (more detail)	Look at the prices in your chosen vault, in the currency you want to buy in.
Step 3. (more detail)	Click the Best Offer gold brick. This loads the order panel with the details of the best offer i.e. with you as a buyer.
Step 4. (more detail)	Adjust your price per kg downwards on the order panel using the arrow key. Now adjust the quantity to the amount you'd like to bid for.
Step 5. (more detail)	When you're happy with your order press the BUY/BID button, and CONFIRM, as for a normal order entry. Then look below for what happens next.

You will probably receive a response in the order panel saying 'Your order is now open'.
As the best bid in the market you will probably see your price come up as the new highest offer green brick for your chosen vault and currency. You are buying, so you are advertising to a seller.
The reason we can only say probably is because you might have entered your order at almost exactly the same time as a seller offering gold at your price, in which case you will already have dealt. Or possibly someone has already bettered your price, in which case you're on the board, but not currently at the best price as a green brick.
One thing that's fun to do now is to view the new market spread. What you just did was narrow the spread on the gold price for the whole world. You can see the effect by pressing the BUY/SELL menu button, then selecting the MARKET SPREAD sub-menu option. You reduced the size of the white gap in your chosen vault and currency.
As usual you can monitor the success of your order from the HISTORY > ORDERS screens. Sometimes it's best to monitor what's happening on the ORDER BOARD itself, because you'll see if anyone is bidding up against you.
If you decide you want to push the purchase through you'll need to withdraw your offer by killing it from the HISTORY > ORDERS screen, and replacing it with one at a higher price.