Then lay it on a flat surface, on it's back. Forget any nonsense you may have heard about guinea pigs having an 'attack of the vapours' like some kind of Victorian Miss every time it is laid on it's back. It can happen but very rarely. I prefer to work standing with the guinea pig at about table height. However, there is no hard and fast rule when it comes to where these examinations are carried out.
Some people find it easier to sit down and have the guinea pig on their laps.
So long as the animal is secure either method will suit. If you have a very observant owner, and most caring owners are, the animal will probably have been brought to you because it has begun to lose weight, or it has been noticed that it has not been chewing it's food with as much vigour as it used to. One owner told me that it looked as though her guinea pig was trying to chew glass, which is about the best description of this symptom. First check the incisor teeth. You will probably find that they impinge at an angle when looked at face on.