These are generally referred to as carpet to carpet pads.
How to put down a rug pad.
Smooth down any air bubbles that might be trapped between the pad and the area rug.
Using the washi tape as a guide roll out your area rug pad and place the area rug directly on top.
Placing the rug pad.
Use a non slip pad.
You must lay the rug pad at the location where you wish to place the rug.
Lay the rug pad out on the floor and measure the length that you determined for the pad down the side of the rug pad.
If using the waffle pad style of carpet pad the waffle side of the carpeting pad should be face up for maximum cushioning and it is cut to fit inside the tackless strips with about gap so that the carpet itself can be attached to the tackless strips and the padding does not interfere with the securing of the carpet.
You may use a rug pad of the same size of the rug.
Before you can put down the rug pad you will need to start by clearing out any furniture that can inhibit your full view of the space since having furniture in the way can keep the rug from laying flat.
Once you have placed the rug pad place the area rug directly on top of this non slip pad.
Placing a pad under a rug not only prevents the rug from slipping on your hardwood floor it also makes the rug more comfortable for walking.
Wait 1 2 days before trimming.
If you can live with a mismatched area rug and rug pad on the floor for a couple of days i strongly recommend this step.
These types of pads are specially designed to help keep an area rug from shifting and rippling over the carpet.
Clean the whole room to ensure that the pad will stay securely in place.
There is a non slip rug pad designed for pretty much every use of an area rug including holding an area rug over broadloom.
It gives time for the area rug s fibers to relax and for the rug pad to settle on the floor.
Another alternative would be to use a rug pad which is slightly smaller but the difference should not exceed 12 square inches.