Developed technique but was not added as co-author I developed a novel technique (from 2016 until my graduation in August 2019) in my area of study as a PhD student. I transferred the technique to another PhD student, which took all of Fall 2018. Today I found that the second student and my (ex)advisor submitted a paper in July (before my graduation), and neither included me as co-author (even though I helped the second student adapt my technique to his problem with significant hand-holding), nor cited my papers documenting the technique.

Today my (ex)advisor sent an email asking me to help transfer my technique to a third student. The third student informed me that the second student (who left the research group for another) is not cooperating, and stalling with excuses of other commitments. Of late, my (ex)advisor has become very toxic to work with, and has lost numerous good grants. What do you advise I do in such a situation?
The proper advice, I think, is to ask for co-authorship on any future papers that use your technique. Point out that you are actually due this for having developed the technique and even more for adapting it to the other students' work. Not for "helping them" adapt it. For adapting it. Nor would I be shy about saying that they made an error in not including you on the recent paper. 

They might just offer an acknowledgement and you can decide if that is enough for you. I don't have a recommendation about that, but think that co-authorship is probably more appropriate. 

And, if they say no, then your decision about helping is also pretty clear, I think. 

I wouldn't make the suggestion quite so strong except that you say the relationship with your advisor is already toxic. You have your own career to think about and your own research goals and needs. You don't need to also do someone else's work without proper attribution.