You must to allow spend of ERC-20 tokens for each token that you have with each Smart contract. This is part of the ERC-20 standard (https://ethereum.org/en/developers/docs/standards/tokens/erc-20/). We execute allowance call with max amount for simplicity.
Let's assume an allowance request appears on UniSwap swap modal window.
Click a "Allow the Uniswap Protocol to use your <Token name>" button
Confirmation window will appear
Navigate to a "Transaction" tab with a section menu
6. Wait until corresponding transaction will change status to 'Completed'
7. You can check an extra information about transaction via information button (left side of each transaction):
8. Transaction Details modal window will appear
9. Clicking by transaction hash link will open Etherscan with selected transactions.
10. Wait until the transaction will mark with the "Completed" status
11. Go to Provide parameters for swap (trade) section and start from a 1 step.