Left thigh for fast acting at breakfast.
Right thigh for Lantus at breakfast.
Left of belly button at lunch.
Right of belly button for dinner.
Slow acting insulin is less sensitive to get into the thighs, where uptake from tissue is slower and depending on your activity level. Smaller injections of fast acting is best in stomach, as you here have more consistent and reliable speed of uptake.
Each area is then having a 'zone' I use for each week day, with pattern like a chess board:
TUE THU SAT
MON WED FRI SUN.
So Monday morning is left thigh, upper side near stomach. Sunday near the knee...
This way there is only one injection per week per site/zone.
For needle, it depends on your own personal preference. But no matter what, your needle-end should be in the fatty tissue just under the skin layer, as that is where the insulin should be placed. If you are really skinny on your legs, than even a 8mm needle can by mistake go into your muscle and that will bruise and hurt. Likewise will it bruise/hurt if you inject the insulin into the epidermis or the dermis. Another important point is that you will bruise and hurt if you use one of the very fine new needles but injection speed is too high, as the force of insulin sprayed out will tear the tissue at the needle tip. Potentially try out one of the 4mm needles on the market for our pens, and hold it vertical in for the injection. This should ensure you get through the dermis and also that you do never get down into the muscle below the subcutaneous fat layer. Take your time (4-5 seconds) before extracting the needle after the injection, to ensure the insulin is not running back out. Most important if you are utilizing relatively small doses of insulin as I do... Also review the thickness of your chosen needle, as thinner clearly makes less damage to skin. I am myself using a 32 Gauge needle, which is among the thinnest on the market. When using this, please just be gentle when injecting, as the insulin stream is narrow but at high speed. (same volume to get out through a smaller hole)