1.Login to your blogger dashboard--> layout- -> Edit HTML
2.Scroll down to where you see tag .
3.Copy below code and paste it just before the tag.
Note:You can change width and height of popup box,colors,.... as your choice.
4.Now save your template.
5.Go to Layout-->Page Elements and click on "Add a gadget".
6.Select "html/java script" and add the code given below and click save.
Note:Replace Your-Popup-Button-Name,Enter-Popup-Title-Here,Enter-Your-Popup-Content-Here with your data.
You are done.
Demo