If you don’t have an envelope to put your card into, here’s one you can download.
This is what the envelope looks like when you print it out onto A4 paper.

Cut round the shape and fold along the lines, with the coloured part to the inside.

Glue the white side of the 2 side tabs with Pritt Stick.

Fold up the bottom flap onto the 2 glued side tabs.

Your finished envelope should look like the top photo.