I have had to wash pigs before. If the pig is very lively you will have to do it in a safe place, I usually use the bath. I put two buckets in the bath, one is filled with warm water, the other is for the piggy to sit in. I put a bit of warm water in the piggy bucket, then add the pig, and gently wet the dirty parts of the pig. Then add shampoo and massage it in.
Piggy goes into the tub itself while I empty out the sudsy bucket, replace with some clean water and put the pig back in to rinse out the shampoo.
After that lots of lovely snuggles with piggy wrapped up in a towel.
If it's the first time your pig has had a bath she will probably not like it much, but once they realise they have been bathed and not drowned, the next time you have to do it, it will be less stressful. Kanga Pig is a mucky girl, but she doesn't get bathed very often, I try to keep her long hair short at the back end so I don't have to put her in the tub. Even so, she knows about bathtime now and takes it all in her stride.