Most freelancers wait far too long to raise their rates, often out of fear that a client will simply leave. In practice, a well-communicated increase loses far fewer clients than people expect, and the ones it does lose are usually the ones paying below market anyway.
Give real notice, at least 30 days for standard clients and longer for retainers, using the rate increase letter generator to keep the message short and confident. You don't need to justify the increase at length, a brief line about rising demand for your time or the cost of doing business is enough.
Time it around a natural boundary when possible, the start of a new year, a new project, or a contract renewal, rather than mid-project on an existing engagement. If a client pushes back, it's fine to hold the line on price while offering a grandfathered rate for a limited window, that gives them room to adjust without you giving up the increase entirely.
Re-run the rate calculator periodically, rates that were fair two years ago are often stale today once you account for experience, demand, and inflation.