Default [Q] Someone explain to me phone flashing and the donor esn part!!

Ok so I want to start flashing phones. I understand the flashing part that isnt the issue for me. I do not understand how to install the donor esn into the flashed phone!

For example
How would I install the donor esn from a sanyo incognito to any sprint phone?
How would I install the donor esn from a metro pcs phone to any sprint phone?

That is seriously what im not getting. I usually google everything but I cannot find a good instructional guide!

Please help me it would be greatly appreciated!

Thanks!!!!