Once installed, you can use the contracts in the library by importing them in your solidity. For example,
pragma solidity ^0.8.0;
import "@relicprotocol/contracts/BirthCertificateVerifier.sol";
import "@relicprotocol/contracts/interfaces/IReliquary.sol";
contract MyContract is BirthCertificateVerifier {
// take Reliquary address as constructor arg, could instead be hardcoded
constructor(IReliquary reliquary) BirthCertificateVerifier(reliquary) { }
function someFunction() external onlyOlderThan(365 days) {
// we know msg.sender's account is at least 1 year old
}
}