The process for manually adding a contract address for ERC20 (ETH) tokens will depend on your particular wallet. If using Metamask, you can select "Assets" tab, scroll all the way to the bottom and click "Add Token". Select "Custom Token" and there you are able to input the token contract address and symbol. After clicking next, Metamask will begin tracking this token's balance for your account.
EDIT: If by "manually" you mean how to achieve this programatically without Metamask's UI, you can use their API through this code
address: tokenAddress, // The address that the token is at.
symbol: TokenSymbol, // A ticker symbol or shorthand, up to 5 chars.
decimals: 18, // The number of decimals in the token