This way of apologising never gets old. Prepare a heartfelt sorry note on fancy paper and put it on their space.
Small surprises like bringing their favourite coffee or flowers can go a long way in showing you care and are truly sorry.
Make her feel special by doing small things like cooking. Cook some favourite food of her to bring a smile on her face.
Respect your partner's boundaries and give them space if they need it. This shows that you value their feelings and are willing to wait until they're ready to talk things through.
When you finally resolve your issues, try to work on things that led to the fight. This will make your bond stronger.
After every fight it is must to tell her that she is special and matters a lot in their life. This is not just an apology but also a mantra which keeps alive your relationship.
A simple sorry not always fix things. Sometimes we have to put efforts and make amends like, take them out on a date or plan something special.