Whole milk, heavy whole cream and/or custard, sweetened condensed milk, and something for the flavor and to to tone down the taste of the whole cream (chocolate powder or anything like that).
These ingredients are quite cheap in my country, they are so-called "basic" products, I don't know if it's the same in yours.
Shake to drink preferably at room temperature or slightly warm, but certainly not cold. Heat expands tissues while cold causes tissues to contract.
try different dosages for the ingredients until you find the one that suits you in taste and texture (more whole milk for a more liquid shake, more cream or custard for a thicker shake). personally, I make a 1L shake (about 0.26 gal for our American friends).
don't forget to shake the mixture well before drinking it.
If you have any question DM me if you tu want.
Best regards
1 month