There are many of ways you can probably think of when applying foundation.
You can use your fingers, foundation brush, sponges but my favorite way is using a Kabuki brush.
I created this video showing how do I apply foundation using a Kabuki Brush.
Products and brushes mention and used on the video:
Foundation:
Le Faerie Foundation
Brushes:
Sigma F82 Round Kabuki Brush
Ellana Flat Top Kabuki Brush
Elf Flat Top Powder Brush
No Named Flat Top Brush from Landmark
Marionnaud N32 Angled Blush Brush