BCD - Karnaugh Map-Logic Diagram-Logic Array
First read what BCD is.

Make a truth table, e.g. 2 in BCD is 0010

Call the digits a, b, c, d... so in this case a = 0, b = 0, c = 1, d = 0
and 8 is 0100: a = 0, b = 1, c = 0, d = 0

Make a table for all possibilities.

From that point it's like handling a Karnaugh diagram like you already know (at least you wrote you are familiar with such diagrams).