A VBA Function returns a value. Assign that value to the function's own name before End Function. After you save the module, the worksheet can call the function by name with input arguments, just as it calls a built-in function.
Keep the function's inputs explicit. Function DoubleCount(n As Double) receives a number; DoubleCount = n * 2 returns the result. In the worksheet, =DoubleCount(B2) then recalculates from B2. Typing the current answer would lose that connection.
A worksheet Function should calculate and return a value. It must not edit another cell or depend on a macro having run first. Test a normal input and an edge case, such as zero or a value exactly at a threshold. Those cases often reveal a missing branch or an incorrect comparison operator.