Sec. of State Pompeo has a big task on his hands and he will need the help of South Korean leadership. But, as this is a team effort and will take a while he should enlist a lot of help from America too.
First Steps First
The first thing I would do is have seminar/meetings (perhaps 10) at various sites around the world on the topic of building N.K. and invite all business leaders who would be interested having agricultural, manufacturing, or services divisions or production sites there. Note: This is not for the infrastructure builders. That's a bit later.
This may require several events as there could be quite a few participants. Each should have a representative of N. Korea to welcome their participation.
These meetings should be seminars in the sense that a plan of development could be laid out to let them know what they're in for and it's costs & benefits.
From the Ground Up
Once companies declare their interest they should be required to pay a fee to show their commitment to participating. It might be $10,000 to weed out the less serious small firms. That would go to the government of N.K. as an indication this project has begun.
Next would be to have meetings with those who have committed to determine the natural resources of N.K. they would need and the man-made infrastructure they would need. This could be electricity, natural gas, water, sewage, buildings, roads, mail service, banking, communications, etc.
Then is the time to invite infrastructure builders to a seminar/meeting similar to the first one. This one would describe to them their role at the beginning of this project, the infrastructure they would be required to build, and the N.K. resources which would be available to them. It would also explain their costs and benefits. They too should pay the participation fee to show their seriousness and to weed out others.
An Important Capitalization Step
The main financial step at this point would be to connect the infrastructure builders with the product-makers. There should be no dependence on N.K. for funding. This is, after all, a capitalist project and not a state-directed and funded project. Product-makers will plan to produce profits and they will use those to pay back the infrastructure-makers via bonds, stocks, etc. They may be required to put up investment funding for the infrastructure-makers to get them going, but it may be possible and better for the infrastructure-makers to simply offer bonds in the open market to raise any capital they need for this.
Role of the North Korean Government at the Start
For N.K. the task is to decide what land they want to invest in this project and which people they would be happy to contribute. They may be military people who are engineers or it may be private citizens who have other skills. The number of N. Koreans who might be used as simple laborers is entirely unknown at this time and would have to be worked out later, if they're needed at all. The land N.K. would want to use should be easily containable for security reasons. Industrial parks where clusters of foreign workers can be watched would probably suit their needs. This sort of thing has been done in other places with success.
A Bright Shining City
I suggest at least one area for development should be for a new city as far from the border with S. Korea as Seoul is from the border. They would be sister cities which would help engender a sense of unity and eventual union (perhaps a century or two down the road).
Summary
This beginning brings together product-makers, infrastructure-builders, investment from around the world, and the Korean governments (North and South). It shouldn't take more than six months to execute since the world knows what is happening and many have already been thinking about their potential role in it..
You may ask, when do we get to 'denuclearization' and the answer is that every step toward integration of N.K. into the world community is part of that process. If N.K. is involved in this peaceful development they won't want war and at some time in the future they will be comfortable as India or Pakistan or other nations which have nuclear weapons, but have no interest in using them or threatening people with them. And, like South Africa, they may feel comfortable enough to shrink their number or eliminate them altogether. In time.