The Extraction prep chart from yasiss will help! To answer your specific Qs..
Remove the skin and core of the mango.
Remove the skin and pit of the avocado.
Yes wash the spinach using a salad spinner or you can just rinse with water and then pat dry with a paper towel
You can either keep on the green top of the strawberry or remove - most often this is already dead and void of any nutritional benefits.
You can put the flax seeds in as is or to ensure they are ground as finely as possible, you could use the milling blade first. If you have any leftover flax meal, be sure to store it in a sealed jar in the fridge to avoid rancidity. Happy Blasting!!