Click "Stake" on the center of the page:
You will see a window with tabs "Spending" and "Wallet":
If you have NFTs on "Spending" go to step 6.
Switch to the "Wallet" and select an NFT to send to "Spending". Click "Send NFT":
Confirm the transaction on the website and in your wallet;
After the transaction is confirmed (1-2 minutes), switch back to Spending;
Select the NFT to lock it for the duration of the Siege:
Ready! The game will start immediately after investing in one of the Factions.