Mango seeds can be harvested from those you buy in the grocery store. I believe they need to be fresh so the next time you buy one take a thin knife and at the front of the husk you should find where you can wedge the knife between the two sides. Being careful not to damage the seed you can pry the two sides apart. Place the seed you find inside place into soil to about half way. keep the soil moist not wet and wait. The leaves will look like avocado leaves but usually the new leaves will have a red tint.

Hi,
Are your custard apple the red variety?
Trini

Yes the red variety but this particular tree produced gigantic 3 to 4 pound fruit.