I've used a smooth white singlefold card from Regal Craft Cards as the base - they have lots of colours available if you fancy another alternative.
Next its just a matter of choosing a square hole punch or die (or ruler and craft knife) to cut some squares - any size will do - these are a little over 1 1/4 inch. Any paper will do too, as long as yoy like it and think it goes together. These are from a Kaiser paper pad, but experiment with old book pages, rainbow colours, embossed papers etc.
Arrange the squares in a way you find pleasing - I've put these on edge, left a little gap between them, and run them off the sides of the card, but there are lots of other choices - a grid, a portrait orientation, a mixture of small and big squares etc.
All that's left is to add a greeting. I've used a pre-made one, but the circle is a good idea as it contrasts with the squares, and stands out a little more because I've popped it onto a foam "sticky fixer" and positioned it approximately a third of the way across, and down - a position that naturally draws the eye.
Give yourself permission to play!!