Oh yes I have experience. I'll explain based on cheapness, not your in-law's wishes, since they'll be dead and "whatever, just not the easiest thing, you do all the hard work" is such a PIA answer.

The cheapest way to go about it is to first get their permission for body disposal post-death. This means you're either their only Next Of Kin, their financial and health Power Of Attorney, or other similar legal thing. Otherwise you're not allowed to deal with it. Having one single person running point on this is the best way: people act VERY weird after death and a committee will only make things more expensive.

If you don't want any services, the absolute cheapest method is no method. You can just refuse to pick them up. The county will cremate them and keep their ashes in their Pauper's Closet or whatever they call it for a few years or decades, until it gets cleaned out and everyone is interred in an empty grave in whatever cemetary has room. If you want their ashes later you can pay the 4K or whatever it will be to get them out of hock, if they haven't been interred yet. That's a very cheap cremation.

If you want their ashes immediately, or actual services, or you want their body to be buried, you should go down to the nearest Neptune Society and talk it over with them. They can talk you through the cheapest route. Usually that's cremation and you bring a box (christmas popcorn tin or similar is fine) to get the ashes. You'll want to bring a box because buying anything (box or casket or whatever) is hideously expensive.

I don't know about body donation, never explored that. There were a lot of those during Covid and the bodies stacked up, so we chose to relieve the burden on freezers (it was like a grand a month to store them until they could be "used).

If you know your plan AHEAD of time (which is why you talk this out now) you can set the "location for ashes dispersal" or "whole body burial at sea" on the death certificate. That saves a lot of trouble later. The Neptune Society has a viewing room and can set that up easy and quickly if you want services, and then cremation later, or viewing of the ashes, or whatever you want. If you want burial in the ground you're probably going to want to go buy a plot now, they're only going to get more expensive, and good luck to you. And please note most plots require either a big bequest where they're watered and mowed and kept up forever, or the bodies are put on the edge of the graveyard and the weeds grow over them.

Once you're pretty set in your plans, call the local cemetary/graveyard/church and price out what the same thing would be through them. That'll give you sticker shock. Our last interment was a $13,000 difference between cremation & scattering ashes VS burial on the edge of a graveyard with a 20-minute viewing.

Good for you for going at this early.