You need to follow Anti-Static Precautions or you will fry out your computer, and turn it into an expensive paperweight.
Your body carries Static electricity. Static will fry out (Short Circuit) the hardware components inside a computer.
You need to de-static your body before working on a computer.
The proper method is to wear an ESD wrist strap, and connect it's alligator clip to a good ground source. (Electro Static Discharge)
I connect to the metal frame of an empty desktop computer case.
Example of an ESD wrist strap,http://ultrastatinc.com/ESD_Wrist_Straps.html
Average cost is around $3 to $6.
Work on a table.
The next option is to have a metal object on the table that you can touch before opening the laptop.
If you leave your laptop in the middle of working on it, be SURE to touch the metal object upon your return.
This metal object should be steel, or iron. A fairly large Knick-Knack, or serving dish, or something similar.
2) Use a mutli-compartment container for the various screws you will remove. Mark each container for the area the screws came out of.
You can use an egg carton in a pinch. Mark each each egg holder.
There may be as many as 60 screws, and 12 different types.
Some look VERY similar to others, but aren't.
DO NOT mix them up.
3) Make notes and drawings as you disassemble, and/or use a camera, or video cam.
Nice to have a record to come back to in case you get interrupted.
Here is the Service Manual you requested,http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&lang=en&cc=us&docIndexId=64179&taskId=101&prodTypeId=321957&prodSeriesId=3442832
On this page go down to the heading >
Quick jump to manuals by category...
,and Left-click on > Service and maintenance information
Now Left-click on > HP Compaq 6720s Notebook PC - Service and Maintenance Guide
This is a PDF file. The computer you are on now has Adobe Reader on it, which uses PDF files.
After you click on the above file name it may take up to 30 seconds before you see anything.
It may take additional time for the file to fully download, after you see the first page.
Suggest you let the file download all the way before looking through it.
Took 4 seconds to see the first page, and 70 additional seconds to fully download just now.
I have a medium speed DSL connection.
Guide that may help you with the DC Power Jack,http://www.laptoprepair101.com/laptop/2007/12/06/dc-power-jack-repair-guide/
One example of a DC Power Jack for an HP Compaq 6720s Notebook PC,http://www.laptopjacks.com/view_part/For-HP-6720s-DC-Power-Jack-HP112600.html