I'll be on my own, same as last few years. NC with family.
My approach is to try to keep busy in the lead up, so the day itself feels like a welcome break when it arrives. Keeping busy for me usually means working, but if not it could be cleaning the house, sorting the garden, anything really.
I'll have some decorations up. Some years it's a traditional tree others a bit more paired down (currently a nutcracker, mouse king and small nativity on the mantle, need to decide if a big or small tree going up). The year I didn't bother with decorations at all I felt worse, like I really was on the outside so that didn't work for me.
Presents. A friend and I usually exchange gifts in advance so I'll likely have a small something to open. However, from about October if I see any little treats I fancy I buy them but instead of using I pop in a bag in the wardrobe and open again on Christmas Day. Doesn't have to be big items and usually a new perfume to try (or an old favourite), books, furry slippers ect and I've often forgotten what's in the bag. It is nice to have treats to open and actually ones you want. Plus also helps with an answer if you later get the ubiquitous 'get anything nice for Christmas?' question.
On Christmas Day I get up, shower, do hair and usually into new or clean PJ's and that's me ready for the day. I'll have various food options in to decide what I fancy. Lunch could be anything from baked bread (the easy ready part-baked rolls), with cheese and salad or pizza and garlic bread or even a 'proper' dinner. Crisps, chocolate, Christmas cake and mince pies will also be in the cupboard and all guilt free!
I'll usually find something to watch on TV (those TV magazines are useful to circle anything of interest in advance) and just relax, maybe read a book or have a browse on line and plan holidays or ideas for days out for next year. This is where keeping busy in the run up pays off for me because it's nice just to sit down and take it easy.
No-one knows I'm home alone and I'm vague if asked about plans, as I don't want nosy questions, pity or people feeling obliged to invite me to their own celebrations.
I'm sorry for those on their own and struggling with it - it can feel overwhelming especially if the first time, and there is a lot of commercial focus on the 'perfect' traditional day. Be kind to yourself, have some little treats, don't compare to what you think others are doing and structure the day in whichever way works for you.