Allow your child to help you choose the decorations as this may help reduce any anxiety.
Bring the tree down from the loft a few days before its due to go up. It could go up in stages, and then and put the decorations up over a few days if that helps.
If going to chose a real tree use visual support and look at the website before going. Take ear defenders as the chainsaw they use can be loud.
Do some Christmas Tree Sequencing activities at home before you decorate the tree.
Consider that real trees can have a strong smell that your child may not like.
Keep some rooms in the home decoration free especially your child's own room if they want.
Let your child be in the room while you put them up. A child that comes home from school with the house looking all different could find that distressing.
Clear baubles can be bought that you add special interest objects in as decorations
Some children may find flashing lights too much so lights that are on continuously with no twinkle may work best.
Consider having a smaller tree that the children can decorate themselves so it doesn't matter what it looks like ;-)