When to add wheat?

[IMALOSERSCUMBAG]

I usually make rum, neutral and UJSSM. Non of which actually require mashing. Time to take the step into actual mashing.

I'm looking for a good explanation of mashing and when to add what and when. I see where we should be adding wheat and barley malts at 150-153 degrees. I would like to understand why and also understand when I should add each type of grain. I don't know if I'm not searching properly or if I'm missing something.