What I've done with this is to buy a big plastic tub from HD. The manifold has to fit entirely inside or you'll make a huge mess for which she'll never forgive you. Wear your oldest, expendable clothes. High cuff chemical resistant gloves, with a pair of thin nitrile gloves underneath. Use a bunch of small paint brushes and disposable cloth rags. The best brushes to use are the cheap ones they sell for applying solder flux, the steel handles do double duty as scrappers. Buy a bag:

https://www.homedepot.com/p/Steel-Core-1-2-in-x-6-1-8-in-Horsehair-Bristle-Shop-Acid-Brushes-144-Piece-75841/308076784

Gasoline is the best solvent, but very dangerous. The advantage is that it's cheap and really cuts the crud. It evaporates quickly, which helps with cleanup. Mineral spirits also works. The good news is that you only have to do this every 150K miles or so.

You won't be able to dump the spent cleaning fluid anywhere, so the best bet is to stick it under the deck until it evaporates. Gloves, clothes, hat, rags, brushes, all bagged and trashed.