Whenever I decide to buy something, I always ask this question - can I use this piece in at least 3 different ways by mixing and matching it with my existing wardrobe? Although there is no rule when it comes to print mixing (if it looks visually appealing, go for it!), but it becomes easier if there is some coherency going on in your outfit. If you are mixing two different prints, make sure either the base color of pieces are matching or similar prints vary in sizes.
ASOS is my favorite place to find great midi skirts. This skirt is one of my oft-repeated skirts, it mixes well with many shirts and blouses I have. In today's outfit, the window pane print is contrasting with the micro-diamond dots, but because of the black and white color scheme , it doesn't look jarring or out of place.
Skirt - ASOS // Similar
Sandals - Zara // Similar (ON SALE!)