Ok, the writing came out backwards in the photo, but you get the gist.
You will need:
Green(ish) washi tape
Other colour washi tape
Stamps or stickers to write merry Christmas
Folded card, any light colour
Optional embellishments, e.g. stars or glitter
Cut out strips of the green, with diagonal edges, in descending size order, ending with a triangle. Stick down your widest strip, around the lower middle of your card. Continue to add the strips, as pictured above, until you get a Christmas tree shape. I used seven strips, but a shorter or taller tree is fine. Then add trunk in 'other colour'. Stamp/sticker beneath and add any embellishments. Write and send!