ANY bid is treated as a "maximum bid" under the automatic a/k/a proxy a/k/a incremental bidding system. It's not an option that you do something special to invoke or can even opt out of; it is simply HOW eBay auctions work (highest bidder wins, but price is calculated primarily from the maximum bid of the second highest bidder, and eBay hides the full amount of the leader's/winner's maximum bid to the extent it exceeds that calculated price). Even if you just bid the minimum, you can end up with a hidden maximum (if the prior leading bid is cancelled). The notion that eBay "automatically bids on your behalf" is an analogy to describe the process for those who want simple answers but results in many misunderstandings of what actually happens in particular situations (such as thinking there is a difference between "regular" and "automatic" bidding).
So, mechanically, you bid the same way you bid now (unless you are using a 1-click bidding button that purports to put in a bid in an amount eBay says, in which case do not use those as they are not only tactically bad--they encourage bidding round numbers in an amount based on the current bid amount--they are outright dangerous in that the amount can change just as you are going to click), except that instead of putting the minimum bid or whatever amount you are now using in the bid box, you put your TRUE maximum you are willing to pay for that item as listed WITHOUT REGARD to what anyone else appears or doesn't appear to be bidding. Since what is needed to beat the second highest bidder, whether s/he bids before or after you, is already figured into the price if you win, there is no need to factor it into your bid amount; all you have to figure out is at what price point you would rather someone else have the item than you having to pay more than that price.
Tip: make your bid a little higher than the round number you are likely to come up with; this increases your chances of being on the winning side of what might otherwise have been a tie (most bidders bid the round number) and decreases the odds of losing to someone else who knows this tip. I strongly advise that you bid ONLY ONCE at your TRUE maximum: there are circumstances where already having bid on the auction can prevent you from rebidding; further bid late in the auction (but in time for the bid to be received at eBay before the end--the countdown is not reliable for timing that within the last few seconds) for several good reasons, including keeping your options open and not using up your limits.
