PE executable packer

I want to code a very simple pe executable packer as a learning exercise though I'm not sure how to go about coding this. I understand I will have to use an existing compression library and will have to code a decompression stub within the packed executable. I have done a lot of reading on the pe file format. My previous experience with c++ has been coding small programs. Does anyone have any advice or recommendations on any tutorials I could follow.
Here's one popular library off the top of my head: http://www.7-zip.org/sdk.html
UPX is an open-source and well known library for exactly this purpose:
http://upx.sourceforge.net/
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