Laying the carpet padding is an important part of the carpeting installation process not only because it can make the carpeting feel more comfortable but it will also have an impact on the durability of the carpet.
How to replace a section of carpet padding.
Install carpet pad so that any seams will be at right angles to those in the carpet.
Install new carpet pad.
Then use a carpet tractor image 2 to press the carpet down onto the seam tape to reinforce the bond.
Use a staple hammer tacker to fasten the carpet pad against the edge of the tack strip.
Now it is time to install the carpet pad.
Older carpet in high traffic areas might be more faded.
If carpet tape is not doing the job you can also use carpet glue to hold the patch in position.
Overlap the edges of the new and old padding to cut straight abutting.
Afterward position something heavy over the area to help set the seam.
Tape neighboring pieces together with 2 to 3 inch masking tape or as directed by the manufacturer.
Place the new pad so the waffled side faces up.
A line or 2 of glue on the surface of the carpet pad will often be sufficient.
Carpet padding has a smooth side and a waffled side.
Keep in mind that if you plan on replacing the carpet the patch will adhere to the padding making it harder to salvage the pad.
If you are only replacing a 6x9 foot area with no seams i would likely just replace the carpet and pad if i could find a usable pattern on roll at lowes or home depot or a remnant at a dealer.