To do this you will need to use a phasor~ to control the playback location, but you will also need to envelope each grain to eliminate clicks (or buzzes, for really small grain sizes.) An example is in patch B09 in the documentation. You'll also need to use one or more samphold~ objects (as shown in example B11.) See also the explanations in the textbook.
From here on out, for extra credit, just go the ``extra mile", makking the patch do something cool in addition to what is specified above...