Cut heart shapes out of your coloured tissue paper. Older children will be able to do this themselves while you are likely to need to do this for younger children.
Cover the paper plate in glue, I've used PVA glue for this but a glue stick would probably work as well and has the benefit of drying quicker.
Stick your coloured heart shapes onto your paper plate.
Cut a tree trunk shape from brown card and stick this on at the base of the tree.