Apply concealer directly onto the spot, and then use a brush or sponge and blend around it. Be sure to use a tapping motion as you want to be careful not to blend any of the product on the spot. You only want to blend around because if you blend over a spot, you take away from the coverage. The key thing with covering spots is layering. When I had acne, I applied a medium coverage all over, used a cream concealer and followed this technique. If I had a particular area of bad skin, I sometimes took a sponge with foundation, and dabbed a second layer over these areas. You want to make sure your concealer is a shade darker and not liquid.
This is a similar method to blemishes. I would recommend layering your foundation. What works for me, is to apply foundation all over, take a brush or sponge and go in with a second layer over areas such as my cheeks and then if anything still needs coverage, I apply concealer. Buffing brushes work great with scars because you can continue to blend with a pressing motion so you avoid blending away product. With scars and blemishes, be careful with where you highlight. You don’t want to brighten these areas and make them more apparent.
The bane of most people’s beauty routines. As mentioned, patting motion is key. Do not rub under your eyes. The area is thinner than the rest of your skin. For severe dark circles, a colour corrector can help and should be applied under the concealer. Most people however, will not need to use this. Use a concealer one shade lighter and be sure to dab directly right under your eyes (where you would apply under eye colour). If you find your concealer is not strong enough, swap from using a liquid and go for a cream. Once applied, I like to let it set on the skin, blend again and then set with powder. The baking method is great-leaving translucent powder to set before blending, but if you have only used a light concealer, this should not be required.
Redness is probably the most common issue people deal with. Firstly, make sure your moisturiser is gentle to avoid any more irritation. Use the blending method of patting. I think the beauty blender is actually great for this. A light hand is needed. From my experience, most medium foundations can cover this fine. Infact, I have even used a tinted moisturiser.
If you have dry skin, you might have the flaky-patchy areas that you cannot seem to blend foundation into. Cream based products work well-this includes cream blushers. Just before applying foundation, smooth some more moisturiser over the patchy skin. As with all the above, you want to be pressing the product on and use a layering technique. Dry skin really needs gentle products with not too much exfoliation.