Force you tongue up to the roof of your mouth and build some pressure then let it out and keep blowing to break the call and draw out the note. Once you get the hang of it is easy to do on about any call if it is tuned right. Play with the amount of pressure you build and the back pressure to get it right. Start pressure will either be too little and the call with not break right off the start or too much and it will break to high and hard. On the back pressure start with a lot and just let off until you find the sweet spot. You can start with a little more and open up a little as the note blows out to tweak the sound.
If you know all this and can do one on other calls and not the SMH then the tune is not right. The SMH is one of my top three calls and I have a wood and acrylic on my lanyard. One of the key notes I tune a call with is the spit and if I an not getting one I can pop right out any time I want one I keep tuning until I get it.