What is the best way for a new 2LT to counsel a platoon sergeant?

Create an atmosphere of trust with respect both directions. I had JO's come in filled with an attitude of superiority and there time with us was difficult. Others came in respecting people but at the same time making their expectations clear. Those people got the support of the enlisted folks and their time was a lot easier and succeeded really well even long after they left us.

The NCO's first priority is to show respect to the Lt in front of the troops so they also show respect . The Lt must understand that he must lead and that is where the NCO comes in and helps the Lt with decisions and getting his feet grounded with all of his troops . A NCO got his stripes for taking care of those under him and a good Lt will also learn the same . When a squad or company fails they all fail learning to pull for each other and all will succeed .
In Vietnam I got a new captain as my boss and I was short with less then 30 days . He gave me a order that I thought was dangerous and tried to tell him how I would do it and he said point blank . Are you refusing a direct order , I kept my mouth shut and headed for "top" the Sgt Major . Top said to me , follow me we walked to the captains office and top said stand at attention Capt. The captain stood at attention and top told him to follow me around and watch what and how I do things and maybe you might make it home also . From that day on I never would under estimate a E-9 and his power .

The 2LT never counsels a SFC. Lol (kidding). First both must understand experience vs inexperienced. The SFC understands the rank situation more than the LT. (Some new LT have been enlisted and possibly went to OCS or Green to Gold). Shan’t matters most is the mutual respect and understanding. We both have a job to do so let’s work together and make it happen. The best officers are trained by the best non commissioned officers. Again I’m kidding. Give each other mutual respect, back each other, this is a crucial point in both careers.

Communications is a two-way street. The initial counseling between platoon leader and platoon sergeant should set mutual expectations. We both have our respective lanes -- responsibilities -- and need to understand that it takes a team to succeed and only one to fail.

Don't listen and you won't become a leader, only a know it all! Learn then lead, they will follow only those who know what will keep them alive! Those that do the job only believe those that walk the walk, not talk the talk! Been there done it. Great leaders follow better leaders!

Very carefully.
As an E-6 in Germany I was a tank commander, platoon sergeant, and platoon leader; I went up and down the ladder for 3 years.
As an E-7 I was a detachment commander.
As an E-8 I was a 1SG.
(something about dragons and being crunchy and tasty with ketchup comes to mind)
;)

As an E-6 I was a Tank Commander, Platoon Sgt, and Platoon Leader; in Germany.
I went up and down the ladder for 3 years.
As an E-7 I was a detachment commander.
But the most fun I had was when I was a 1SG and 2 lts tried to tell me what to do and how to do it.
;)

There are a ton of fantastic responses to this thread. I'll add my input and apologies if double (or triple) tapping as I'm sure I did not read all the comments.
1) As officers, we have the "Big R" responsibility. Plain and simple. My first day deployed as a 2LT, my Company Commander said I had the responsibility for all the good, bad, and ugly. Period! And that needs to be loud and clear on the 4856.
2) Now, the nuance. Each Officer and their Senior Enlisted counterpart must work as a team and joined at the hip Battle Buddies. The PL and PSG, the CO CDR and 1SG, BN CDR and CSM, and so on. And same with the Staff Sections. As a BDE S-1, I should have the same relationship with my MSG. That partnership should be explained in the 4856, working together is the norm, counsel and mentor each other. The Senior Enlisted should be ok with grabbing the collar before that officer walks off the cliff (My 1SG did this to me a couple times as a CO CDR). But at the end of the day, the final decision rests with that officer. And that Senior NCO should both respect AND expect that!

I showed them the counseling I received from my commander and explained my expectations. I used that to build their counseling statement so there were not any issues when I presented it.

Probably a little different for me than most. I commissioned at 35 through ROTC after 12 years prior-enlisted experience making it up to E-6 myself. I was probably one of the few 2LTs older than their PSG. We were actually a month apart, so the same age, but I was older, as he would constantly remind me later. :)

I did my research on the guy, talked to his former CDR, XO, and PL and went into the meeting with respect. Every PSG is going manage their troops differently. (Yes, the troops belong to the PSG, not you. They spend waaay more time with them than you will.) He said sir a lot and kept me from looking like an idiot (probably a whole additional duty in itself) which I greatly appreciated. Hopefully by the time I take command, he'll be my 1SG.