Wa alaikum salaam wa rahmatullah,
I came across a method during my tajweed studies that I hope you and others will find helpful, inshaAllah.
Say Ahhh for a few seconds, then while doing that, squeeze your nostrils closed with 2 fingers. Open and close your nostrils many times and you will notice that it doesn't affect the sound coming out of your mouth. This is because the sound of Ahh Ooo and many other sounds don't originate from the nose.
But now try to do the same exercise by humming and trying to close off your nostrils, you'll notice it muffles the sound because humming (as you can see how the word is spelled) has an m (meem) sound in it. The same goes with words that have the letter n (nuun).
This realization helped me a lot when reciting. If you hold your nose while reciting Surah Al-fatihah for example you'll notice that when you come to words that have meem or nuun in them , they will be hard to pronounce since these letters originate from the nasal cavity. This will also help you realize when your nasalizing letters that shouldn't be nasalized (for me I realized I was nasalzing the du in Alhamdulillah).
You can watch a detailed video ghunnah at Quranic Webinars That course has over 100 videos that take you from the basics of pronouncing letters all the way through basic tajweed rules. A perfect series for those who want to start learning tajweed. But remember, its important to review what you learn with someone who knows proper tajweed who can point out your mistakes and fix them.