It can be useful to practise syringing with your guinea pigs before you have the need for medicating, so your guinea pigs are used to the procedure for whenever you have to medicate or syringe feed. Use water or the juice from veg (like cucumber) for the practice.
Here is a video from such a practice run, courtesy and with permission of Cavy Corner sanctuary:
Here is a video from such a practice run, courtesy and with permission of Cavy Corner sanctuary: