You can find incremental auctions along with regular auctions after the scheduled publications of yearly products. To participate in the auctions you must follow the same process to place your bids. However, there are some relevant differences in the details page of incremental products.
Incremental Auction Details Page
The details page of all auctions which are part of an incremental scenario will look like this:
Most importantly, it bears a red banner which contains a brief explanation about the nature and process of incremental scenarios:
Secondly, a pop-up will be shown when you press the "Place bid" button during the participation in an incremental auction.
This pop-up will remind you that the auction is part of an incremental scenario and will display the other auctions that are part of said scenario.
Note: if you want to be sure to receive capacity for a certain runtime, you would have to place a bid for all offer levels at the relevant network point.