First apply the foundation and then apply the concealer otherwise you may Smudge it away when you blend your base, Gently dab with pad of ring finger or consider a concealer brush for clean, effective blending
Dot on and blend gently for seamless coverage. You can also dot some concealer over foundation, before powder, for added coverage. Choose the right color. Without the right color of concealer, you may highlight rather than conceal your trouble spots.