How should I counsel a soldier involved in an accident driving a military vehicle on base, but the accident is still being investigated?

Just write what they failed to do. Anyone can get in an accident that's not their own fault. Identify what errors in judgment they made, or preparations they may have failed to make. If you can't identify anything they did wrong, then refer it back to the person telling you to write it. Tell them to write the counseling.

Ok, so the got into a car accident. It happens. Start by verifying that the troop has a valid drivers license and has the necessary training to get a military drivers license. Next get a copy of the accident report. Or the officers report who was on scene. Then ask the troop to recount what happened, word for word. Based on this you should be able to get a fairly clear idea of what happened.

Based on the above, then sit down with the troop and have a loosely written counseling, you will later go back and type it up, and lay out that driving a privilege, and a serious responsibility and that involvement of a piece of government property in that is a serious event and should command decide, there could be UCMJ action involved. Lay out any errors that the troop made, if any but also lay out their positives that they made too.

Stress safety in your counseling. Let your counseling state the facts and observations and follow the regulations.

Lastly, once you have it typed up, I would avail myself of my CoC open door policy and speak to the 1SGT unless he was the one that directed you to counsel the troop. Show the counseling to the 1SGT and get his or her opinion on the matter. Then providing they are ok, present said counseling to the troop, and have them sign, date and add any remarks. Done.

This allows you to be thorough and neutral without railroading a troop and keep your CoC in the loop and for input. It may be a case of a NCO getting over zealous in recommendation for UCMJ.

Simple answer is you don't. Wait until the investigation is over. One of the most annoying things we see at the MP station is unit's jumping ahead such as getting an IO when the investigation is still open by the MPs. Let the MPs do their job, close the case, then take it from there.

Be honest. Tell them that they need to wait until the investigation is completed.

Remember that a counseling statement is simply a record of a conversation.
My recommendation: Go in with a handwritten bullet list of things to discuss (in this case, likely the 5 Ws). Have a CONVERSATION about what went wrong and why. Discuss what can be done in the future to prevent a similar situation, not just for this Soldier, but for all Soldiers.
Then, based on your discussion (including preventive measures) determine a course of action.
Do NOT make promises, deals, or threats that you do not have authority for (i.e. UCMJ will or will not happen, driving priveleges will or will not be revoked, etc.), but DO lay out possible consequences.
Write on your hand written form responses and pertinent data as the conversation proceeds.

After all of that is done, then go back and type and "prettify" the form. Go back over it with the Soldier to ensure it properly encapsulates the conversation.

This is how almost ALL counseling forms should be done, honestly. But it is especially important in situations where you may not have all of the facts or where the Soldier needs a COUNSELOR not a judge.

I met that guy in the late 80's at Ft Bliss. He was drag on a group of about 5 vehicles, there were no road guards and He blew through a red light and took out a rental I was driving a couple of Warrants around in. Mp's didn't give Me any kind of citation.

I would at LEAST speak to the MP Traffic Section to determine if they plan to cite him. Actually, at the unit level, nothing should be done until the accident report is received (DA Form 3975). It would sure look bad if his accident wasn't his fault, or even if so, there were mitigating circumstances - like an animal that might cause a driver to swerve.
I would suggest to whoever "asked" you to write the statement to hold off until the FACTS are KNOWN.

In this case, I would quote Sgt Friday, from "Dragnet"
"Just the facts, ma'am. Just the facts."
Focus on the facts.

You need to talk to your company Gunny or your Army equivalent before I’d do anything or put anything in writing. If you get it wrong it can’t be undone, I’d look to Sr NCO Leadership here.

We had an accident at Bragg with blackout lights only, a Cpl was skating and catching some Zs, and the SSGT directing the convoy move failed to see him and he was run over and killed by a 10 ton. Brought up the SSGT on charges and the driver as well, I was EAS before this concluded but the SSGT was about to have a nervous breakdown , his hair was falling out. How the driver was charged is senseless, if anyone’s been under blackout lights you know you literally can’t see shit. That skating Cpl was at fault.

I think Col Grierson is giving good advice. Be careful. What kind of investigation is ongoing? Were there injuries? There should be a 15-6 underway and you shouldn't do anything to muddy the water for those investigating the incident. I think you need to exercise some tactical patience and lets this develop