The process to join a Polkadot parachain auction on Kraken is very straight forward. This is one of the main reason to join a Polkadot parachain auction via a centralized exchange, like Kraken or Binance.

Here, we have created a step-by-step guide. 

1. Sign up for a Kraken account
2. Deposit or buy Kusama (KSM) or Polkadot (DOT)
3. Navigate to Earn > Parachains (See screenshot below)
4. Select the project you would like to support
5. Enter amount of Kusama KSM or Polkadot DOT to contribute
6. Select contribute and confirm (see the second screenshot)

Source: https://www.cryptocoinzone.com/passive-income/polkadot-parachain-auctions-on-kraken-and-binance-explained/