One of the first rules of estimating is that you shouldn't estimate work that you don't understand. Estimating work that you don't understand is little more than guesswork.
So what do you do when you're asked to create an estimate for work that's unfamiliar to you? One option is to see who else you can talk to. Just because you're unfamiliar with the work doesn't mean that everyone else is. So the first question you might as is "Does someone else have expertise in this area?"
As the name implies, you can seek out experts who can help you estimate the work. This could be an outside resource, but you may also find someone within your company that can help. If this is the first time you've used a new technology, you may need the help of an outside research firm to provide estimating information. Many times these estimates are based on what other companies in the industry are experiencing. You might also go to the product vendor. This technology may be new to you, but perhaps there have been hundreds of companies that have implemented the technology before. The vendor may be able to provide information on what it will take to implement the technology in your organization.
The Delphi technique is similar to expert opinion, except that you use multiple experts and try to reach an estimating consensus among them. You need to identify two or more people that you consider to be experts in the type of work you are estimating. Then send them the relevant information they need to understand the work. They should send you back an estimate of the effort, along with any assumptions, risks, etc. that they identify.
If the estimates are relatively close, you should feel pretty confident using an average of their input for your final estimate. However, you may find that the estimates are not close to each other, or that some of the estimates are close to each other but other estimates are not. If that happens, send all the estimates, including the assumptions and risks, back out to the experts for review. Ask the experts to consider the estimates, risks, assumptions, etc. of the other estimators. Then ask each of them to provide a second estimate of the work. Hopefully, you will find the various estimates closer now. Based on a common set of assumptions and risks, the experts should be able to reach a consensus estimate.
If the experts can't agree on the estimate (within a reasonable percentage), then the Delphi approach provides you with guidance on what you can do next. In some instances, you might need to seek more experts to drive to a consensus estimate. You may be able to average out all of the estimates. You might need to just drop any estimate that is far off from the others. There is no one right way to handle a disagreement among experts, but the Delphi technique gives you guidance on what you should consider.