Hi Mark,
The master re-finisher in our group is our esteemed host, Brian Loree. He restores 'haubes professionally.
I wade in because I know a bit and want to learn more.
If you do want to undertake a refinishing job yourself, do so with great care and much deliberation. If the helmet is a good piece it may be worth sending Brian's way.
If on the other hand, it is sort of an average piece and pretty rough around the edges, it can provided a useful learning situation. This assumes that the original finish is not worth saving so you are just as well to start from scratch.
Quality finishing is not for the faint of heart. It requires patience and considerable skill to get right. Whatever you do, go slow.
First off, take note of any paper tags or if the liner includes a silk skullcap (for officers). If so, you need to be careful with your cleaning and leather-conditioning, not to damage these parts.
Then, you need to remove all metalware. Be careful doing this as the wire tabs on many of the old mounting brads may be very brittle. Once you get them off, you can re-anneal them with a gas torch but, again, go slow, you will discolour the face of the piece if you overheat it.
Next comes cleaning the leather. A good, mild soap (Murphy's Oil Soap, a good saddle soap, even a glycerine-based facial soap are good alternatives) and a minimal amount of warm water. I would be inclined to us a not-too-stiff brush or a coarse towel to work loose any crud.
Let the cleaned helmet dry naturally. Using high heat will lead to shrinkage.
Next, it may be worthwhile to condition the leather. Lexol is an excellent product.
Now, you must decide whether you are going for a "Utility" finish or a super deluxe "Parade" finish.
Your basic utility finish can simply be a number of well-buffed coats of boot polish.
A high gloss, parade finish is more akin to French Polishing fine furniture. If there is a furniture finisher in your area, he may be able to help you out with the necessary supplies. As for technique, he might be willing to give you a few pointers. Otherwise, Google and your neighbourhood library can help you out.
If you and your helmet survive all of this, please post some before and after pictures for us.
Good luck,
Laurie