At what point while building/testing Logistic Regression do I perform the PCA or MCA?

[deadendstreet2]

I'm really confused on this. My coursework doesn't go over PCA or MCA, which is not a huge deal because there are plenty of resources online, but it's required on our project. But what I'm struggling with is how do I apply this when building a Logistic regression model or a multiple linear regression model. I actually have the code for the PCA written, just confused on this part. Do I do it before I do the split testing? After before I validate the results?

Thanks so much for any suggestions. If you just have a good link, that works as well.