Okay, apart from the probably improper spelling of the word above, what do you do with the lips. I’m a clarinetist and have my upper teeth ON the mouthpiece above but covering lower teeth that would otherwise touch the reed. How do you “mouth” the mouthpiece on a tin whistle?
Hi Kye … No, you don’t want your teeth on the mouthpiece. Just purse your lips slightly, but relaxed, and insert the tip of the mouthpiece a few millimeters, resting gently on your lower lip.
Here’s a write-up I did a while back on whistle embouchure:
https://forums.chiffandfipple.com/t/whistle-embouchure/56403/39
Apart from asking us all to write a theses like MTGuru’s excellent one, all I can say is that playing a reeded instrument and the back pressure, makes my head feel like popping!
I’m trying to learn how to play the guanzi (like an open hole oboe) - it requires a tighter embouchure that you would be familiar with, than what I’m familiar with (open keyed flute).
A tin whistle … like a recorder … has a fipple or a labium. Only children ‘teeth’ it like a milk bottle. That makes me nervous about buying second hand whistles which sellers claim are ‘a relic of my childhood’ 
Anyway - there was an old (snobbish) school way of thinking, which dismissed both on account of the tonal variation being limited, since the player usually forms a seal around the fipple or mouthpiece, and blows either gentle pressure, or rising pressure, high pressure, in order to control the dynamics.
This is probably important as a first step - then, dynamic control, comes from fingering the open holes. You can bend notes by using open hole techniques which you won’t be doing with a clarinet. Equally, you can learn cross-fingering to play notes which present problems for the non-keyed whistle. Generally - relax! The mouth has to be way more relaxed, If you were asking how to play the open hole flute from a clarinet, I’d feel really sorry for you, only not really as much as for me, since I’ve had to endure a month of losing my embouchure and dealing with numb lips, from puffing, sucking, teething, squeezing, mouthing, and doing anything I can with the reed of the guanzi, to try to control it 
You’re going to find it much easier moving to a tin whistle
Yeah, what Guru said. Just lay the mouthpiece on your lower lip and gently lower the upper lip atop it.
I read here awhile ago that it’s possible to grip the beak of the whistle with one’s lips
and use this to control the air flow and so the whistle’s tone, at least somewhat more.
I’ve been doing this and it seems to work. One grips the mouthpiece very close to its end,
so little of it is in your mouth–just what you need to grip it. Playing flute gives one
the muscles to do this, I think, and so control the tone.
Beginners probably should follow the excellent instructions above but flooters may find
it worth playing with. FWIW.
Perhaps this is what MT is getting at when he writes (linked to above):
Aperture: The tightness of the lips, allowing control of the relative air pressure vs. air volume going into the instrument. Requires a shallow depth. Particularly useful for taming the upper notes of a shrill whistle, and can also be a side-effect of angle and elevation, as mentioned above.