You don’t have to believe in life after death to commemorate the death of a deceased love one, as essentially it entails to honour and celebrate what they were in their living, and these methods will be significant to you and your mom and what she was for you, and not some official or spiritual rule.
There are some good suggestions here. Take out a photo album, or whatever else inspires and moves good memories for you. Something you guys did together, remembering certain events.
Sure it will be sad at the same time, but that’s why commemoration and sinking back into good, or maybe even bad memories helps to deal with it in the long run.
My dad died in 2002 and whenever I miss him too much, I put on The Beatles for like a whole afternoon. It was his favourite band, and whenever I hear most of their songs it springs up some good memories all the way back from when I was little to the last of my teenage years, since he always had em blaring. To me, that’s who my dad was, an old cheerful hippie who let nothing get him down, and this is my own way of celebrating him, even years after he’s gone.
So you can try and find you own personal thing that was also your mom’s and use this as commemoration. Don’t force yourself to find some symbolism, that’s not what it’s about, essentially when you reminisce over good times, whether it be through reruns of a TV show you guys watched together, a photo album, some book she read to you when you were little, going to a place she liked, whatever else, you’ll find some significant way to celebrate what she means to you.