Three ways in, and one chess.com quirk worth knowing about.
On the Home or Analyze page, pick chess.com or Lichess and enter a username. For chess.com, this pulls your last three months of games directly from chess.com's own public API. For Lichess, it pulls your 50 most recent rapid, blitz, bullet, and classical games. Either way, pick the specific game you want from the list that shows up.
A direct Lichess game URL (like lichess.org/AbCdEfGh) can be pasted straight into the Analyze box and it'll be fetched and loaded automatically — no need to go through the username list if you already have the link to a specific game.
Here's the one thing worth knowing: chess.com game links (chess.com/game/...) can't be pasted the same way — chess.com blocks direct fetching of individual games by other sites. If you have a specific chess.com game link rather than just a username, there are two workarounds: use the username loader and find that same game in your recent games list, or open the game on chess.com itself, use Share → Copy PGN, and paste the raw PGN text directly.
Any PGN text — copied from chess.com, Lichess, a database, or typed by hand — can be pasted directly into the Analyze box and it'll be parsed and analysed the same as an imported game. This is also the most reliable option if a username lookup isn't working for some reason, since it doesn't depend on either site's API being reachable.
The three-month / 50-game windows aren't an artificial limit — they're simply how much recent history chess.com's and Lichess's own public APIs return by default for a straightforward lookup. Older games are still reachable individually: a Lichess link works for any game regardless of age, and a copied PGN works for anything, from any source, at any point in the past.
Once a game is loaded, head to Analyze to run it through Stockfish, or see how to read the evaluation once results come in.