This guide is not a replacement for the official guide here. You should go to the official and follow all the steps there. This guide is a complementary to the official guide, with detailed step-by-step using one of the methods, that hopefully could help you to claim your DOTs if you have any difficulty by following the steps in the official guide.
Key in your Ethereum Address (That you used for participate in Polkadot reverse dutch auction back in October 2017) to check if the address own any DOTs.
Do it here in Step 1 of https://claims.polkadot.network/
We have to have an address in Polkadot network to be able to receive the DOTs. There are 4 methods mentioned in https://claims.polkadot.network/. Here we are going to discuss about polkadot.js browser plugin only, because it has the balance of user experience and security. Also if you have been nominate staking in Kusama Network, you are probably already using it.
The source code of the browser plugin is here https://github.com/polkadot-js/extension
Click on the below link to install the browser plugin.
A) After you installed the browser plugin, you should see an Orange Polkadot logo on your browser as below, click on it, you should see the screen as below.
B) Click on Options, and change the display addresses format to "Polkadot(live)". Click Back
C) Under Add Accounts, click "I want to create a new account with a new seed"
D) Give it a descriptive name (display name locally) and then set and confirm the password. You can save the mnemonic seed safely. If not later, you should export the json file then keep it safely and remember the password. You should backup both your mnemonic seed and json file. Mnemonic seed should NOT be exposed to anyone as shown in the screen capture (This would not be used in anywhere). Click "Add the account with the generated seed".
E) To copy your address, which start with 1, double-click on the address, right-click then select copy. Drag your mouse to select then copy will copy truncated address.
Please note that if your address is not start with 1, you have to make sure your address display format is for Polkadot, not others. (5 is for general substrate, while E, F, H are for Kusama Network)
You need to use MyCrypto to claim your DOTs. The most secure way is to download the desktop version of MyCrypto.
They have desktop version for Mac, Windows and Linux. The latest version is 1.7.7
A) Launch your MyCrypto App, Click on "Contracts" on left menu. Under "Select Existing Contract" select Custom. Under "Contract Address", fill in 0xa2CBa0190290aF37b7e154AEdB06d16100Ff5907 (Refer to official guide's claim contract address to verify). Copy official guide's claim contract ABI to the text box under "ABI/JSON Interface". Click "Access" after you have filled up all required fields.
B) In the "Read/Write Contract" drop down box, select claim. For _eth address drop down box, you need to key in your Ethereum Address that own DOTs ERC20 tokens (the Ethereum address that you used to participate in Polkadot reverse dutch auction back in October 2017). For _pubkey bytes32, refer to the screen capture below.
C) You need to copy the Polkadot address generated above (started with 1) in step 2, and go to official guide to get your public key. Your public key is being circled in red ink.
D) Fill in your public key you got from above step, to the text box under _pubkey bytes32 as below.
E) Unlock your ethereum account in MyCrypto, and send this transaction. Set 200,000 for gas limit. Click "Write" button in MyCrypto, then confirm the transaction by clicking "Sign Transaction".
Verify your claim in Step 4 of official guide as below. The address below does not own any DOTs, so your result will be different.