I have used the UWP as my terraforming base.
Each 100 years times the world SIZE divided by TL move the ATM, HYD, or TEMP by one unit (up or down Designers choice). At TL10, it takes several centuries to terraform a moderate world. Cost is 1 TRILLION credits per World SIZE point. The bigger the world, the harder it is to terraform...
All the details about adding nitrogen, etc. are part of the UWP move.
One thing of note - worlds want to return to their original condition, so it takes MONEY and equipment (investment) to maintain a world at some new UWP. Think weather satellites, the occasional icy asteroid impact to keep the HYD level up, or even massive atmosphere converters that have to keep running. After a few thousand years, the world will likely be able to balance itself in the new UWP configuration.
It makes it expensive and a long-term project, but in certain situations it might be worth it. In most Traveller settings, by TL12 or so, they don't bother to terraform since so many other habitable worlds are available. SO, you might have a cluster of previously terraformed worlds around a race's homeworld, but farther away, not so much.
My friends call me Richard.
You can call me Sir.