First, create or find a Prezi you would like to embed in a Google Sites Page.
Find the 'SHARE' button at the bottom right of the Prezi.
Find the </> Embed option. Select it.
Notice the few options you have - to set the width and height, and to allow viewers to pan/zoom or not. TIP: for a standard Google Sites page, the defualt 550 may be enough. You can go back and tweak it if needed. Note that any changes you make to the width/height changes the code generated by Prezi. You will need to grab the new code anytime you tweak the H/W.
Click 'Copy Code to clipboard' button in Prezi.
Go to your Google Site and open the page where you wish to embed the Prezi (or create a new page).
THIS IS IMPORTANT: On the Sites Page, place your cursor where you want the Prezi to be. Then, in the text editing menu, find the <html> tool. Click it.
Things look a little weird now - don't panic. Just PASTE the code you copied from Prezi. Then click UPDATE.
You may get a pop-up warning. Don't panic. Just click OK.
SAVE your Google Sites Page to view.
If you have to resize the Prezi to fit your Sites page, go back to Step 1. When you get to step 6, just open the <html> tool and remove the old code and add the new code from Prezi.