You have perhaps, by now, thought okay, but how exactly do I set these special permission bits?

This is simple: you use the chmod(1) command (or system call); this table shows how chmod can be used to set the setuid/setgid permission bits:

As a trivial example, take a simple Hello, world C program and compile it:

gcc hello.c -o hello

Now we set the setuid bit, then remove it, and set the setgid bit instead (in one operation: via the u-s,g+s parameter to chmod), then remove the setgid bit, all the while long-listing the binary executable so that the permissions can be seen:

$ ls -l hello
-rwxrwxr-x 1 seawolf seawolf 8336 Feb 17 19:02 hello
$ chmod u+s hello ; ls -l hello
-rwsrwxr-x 1 seawolf seawolf 8336 Feb 17 19:02 hello
$ chmod u-s,g+s hello ; ls -l hello
-rwxrwsr-x 1 seawolf seawolf 8336 Feb 17 19:02 hello
$ chmod g-s hello ; ls -l hello
-rwxrwxr-x 1 seawolf seawolf 8336 Feb 17 19:02 hello
$

(As this Hello, world program just trivially prints to stdout and nothing more, the setuid/setgid bits have no perceived effect.)