With a lot of towels and be prepared to be wetter than your pig.
I use piggie shampoo from pets at home and an old washing up bowl on the floor (If they leap out they don't fall far). I get a jug of warm water ready and pin down the pig in the bath whilst wetting him and then lather him up. He gets rinsed an then a good towel dry followed by a blow dry on a low heat (keep your hand between the hair dryer and the pig). He usually tries to escape a lot so I end up soaked
Mine are bathed every 3-6 weeks depending on their stinkiness!
GP1 is starting to sport a GreaseSpot 'Tail' so it's time for his annual wash
DD and I wash them in the bath- they don't enjoy the experience and they always threaten to leap out. I'm not sure if they could hoist their carcasses out of the bath but we don't want to risk it (previous ,more determined guineas have done ).
They smell lovely afterwards then DD and I spend ages drying them, changing our clothes and sorting out the wet towels.
My sister used to wash her long haired piggies. Used baby shampoo or special animal shampoo. Wet, lather and rinse either with a jug or the shower attachment if you have a shower over a bath. They shake like dogs so you will probably get wet. Then used to wrap them in old clean terry nappies or use a hair dryer on a low heat on them. in summer used to dry them in the sun.
Make sure the water is shallow enough to keep their faces and especially their ears out of water. I had one Guinea years ago who loved to swim. We;d fill the bath, put a folded up towel 'island' in the bath and she just went Her mum-pig would skin anyone who took her near the bath.
My boars think they look like lions with their great ruffs of fur and slinky bodies. Truth is, they look like little rats
DBoar 1 is a Texel (of sorts) so a bit high maintenance. He has a monthly-ish haircut and bi-monthly shower in the bathroom sink under the mixer tap. DBoar 2 is mostly short hair, so gets away with a shower about every 4 months. I just tend to use Head & Shoulders 2 in 1 with an extra conditioner, and they smell divine for at least a week.