I thought initially along the lines of the super smart plan, months in the making. Every contingency covered, depending on how things panned out. I've mentioned mushroom gravy brought by a (now deceased) guest in the orange cake container etc.
I now lean towards her being "handed" the plan via reading it in a book. We've seen "The Chalet" mentioned, but I favour Bonnie Garmus' "Lessons in Chemistry". All it really required was the procuring of the death caps and hiding them in a suitable meal. It totally relied on them all dying before too much information could be revealed. Then, as the book said, just play the dumb housewife who has made a terrible mistake.
Some thinking had to go into obtaining the death caps. Even that was poorly done if she was smart. Do the researching at the library, not on her own devices. Don't take her phone with her to incriminating locations. Don't post stuff about dehydrating anything, no-one needed to know that.
The meal itself was well thought out, but it relied upon dead people telling no tales. The individual portions and colored plates were great, until Simon knew about it.
When the plan revolved around playing the dumb housewife in the aftermath, she wasn't well equipped to deal with a situation that called for anything but dumb. Given the cards she was dealt, she played the best hand she could, by retrofitting her story (lies) to the evidence. Thankfully, 12 of her peers saw straight through her bungling.