The main rules to an even blend are:
Only double the visual length of the nat. hair
Make sure you point cut all the nat. hair prior to application for a softer edge.
When finishing your last solid row, add one-two rows of closely scattered ext and then finish with a scattered row of micro ext. So if 'X' represents an ext. , 'o' is a micro ext and '-' is a space. it would look like this (if this was the back of someones head!):
o---o---o---o---o---o---o micro scattered row
X--X--X--X--X--X--X--X--X closely scattered row
XXXXXXXXXXXXXXXXXXXX solid row
XXXXXXXXXXXXXXXXXXXX
Try to use varing shades to to blend with the nat. hair
When trying to blend in with cutting, just 'walk' your scissors into the ext to create a layered look. Also, try to remember that ext. always look better after the first wash. I call my clients in a week later just to 'tweak' the cut and check them over.
Hope this is helpful.