It’s done almost identical as in the introduction user guide, but simply replace the instance of the function block that you want to test with the call to the function instead.
Assume we have a function:

FUNCTION F_Sum
VAR_INPUT
    one : UINT;
    two : UINT;
END_VAR

F_Sum := one + two;

Then the test would look like following:

METHOD TwoPlusTwoEqualsFour
VAR
    Result : UINT;
    ExpectedSum : UINT := 4;
END_VAR

TEST('TwoPlusTwoEqualsFour');

Result := F_Sum(one := 2, two := 2);

AssertEquals(Expected := ExpectedSum,
             Actual := Result,
             Message := 'The calculation is not correct');

TEST_FINISHED();