Brushing: Brushing guinea pigs is really good it keeps their coat clean and healthy. They also typically enjoy being brushed. And it helps with their shedding seasons (yes guinea pigs shed!). I would recommend brushing guinea pigs once a week with a plastic slicker brush or a soft bristle brush. You can typically find plastic slicker brushes in the small pet section of most pet stores. I definitely would not recommend using a metal slicker brush. Guinea pigs have very delicate skin. And if you have a long haired guinea pig breed they will need to be brushed very frequently and may need to be trimmed.
Clipping pet nails in rarely easy, but I have to say guinea pigs are the easiest. Only use nail clippers meant for small pets like ferrets, guinea pigs, rabbits, etc. They will be very small scissor style. Small cat ones in the same style can also be used too.
These ones work great.
These ones are cute and they work okay, but not as good as the ones shown above. I also like using the "Whisker City" scissor style cat clippers that are basically the same thing as both of these but they work super well.
Now, you need to be very very careful not to cut the quick, or nerves and blood vessels in the nail. A lot of guinea pigs have somewhat clear nails that allow you to see the quick, but some don't. My trick for pets with dark nails is to very very gently squeeze the clippers down on the part of the nail you want to cut (without cutting it yet!). If they flinch, or seem surprised, that probably means that you are too close to the quick. If you don't get any reaction it is probably safe to say that you can go ahead and cut. Another tip is to only take a very small part of at a time, and if you clip the nails weekly or one every two weeks you will probably only have to take of a tiny bit. You can also try holding a flashlight under the nail but I find that that doesn't always work. To cut the guinea pigs nails you can try setting them down in your lap or on a table or the floor and try clipping them that way, or you can hold them with hand and cut the nails with the other. (Just make sure they can't fall if they struggle). If they put up a struggle, you can get a friend to hold them for you, or take them to a vet to get their nails clipped.
and... if all tips fail, and you do cut the quick... don't worry!!! I have tips for that too!
If you do cut the quick it is going to bleed. To stop the bleeding you can use special Qwik stop or styptic powder(found at most pet stores near where they keep nail clipping supplies for dogs) this will stop the bleeding. An alternative is to use some corn starch on a cloth and hold it to the nail, this will also help stop the bleeding.
Bathing: I generally recommend bathing a guinea pig once or twice a year. Here is how I do it. First make sure you have shampoo that says that is specifically made for guinea pigs. I usually use Super Pet Squeaky Clean Critter Shampoo made for small animals like rabbits and guinea pigs.
Note: This picture also shows the "spray fragrance" and the "instant shampoo" I don't recommend those because guinea pigs groom themselves, so they would end up ingesting the stuff, and they aren't really necessary. Stick to the regular shampoo.
You can use a sink or a tub, whichever you want. Place a towel on the bottom so your pet won't slip. Then get the guinea pig wet with warm (but not hot!) water. Be careful to avoid the face and the ears! Then get a small amount of shampoo and work in into their fur, once again avoiding the face and ears. Then simply rinse it off very well. Then make sure that your guinea pig is all the way dried. You can dry them with a towel, but you can also use a hair dryer on the lowest heat and the lowest speed. (For long haired breeds you will probably have to use a hair dryer to get them dry). And then you are done! Just make sure to try to keep them as relaxed and calm as possible through the whole process. Some guinea pigs will get very stressed out.