Simple Chrome Extension

Hi. I need a simple chrome extension that once installed it inject simple JS code in each site that user visit.
We can test with a simple alert() in JS. so I will installed the extension and I should see the alert when I visited any site. If you have experience in chrome extension you can do it in little hours.

two questions:
1) how much time will take ?
2) which manifest version you will use ?

thanks you !