Where did the idea of doing a Seder on Tu b'Shvat come from? Why a Seder, not just the eating of fruit? Why do we call it a Seder if there is no formalized way of doing it?
See on the Seforim blog:
There are those who claim the custom to celebrate Tu-beshevat as a holiday is based upon the book Hemdat Yamim. This book, according to many, was either written by Nathan of Gaza (Shabbati Zvi's "prophet") or one of follower of Shabbati Zvi. (This is contrary to the assertion in the Philogos that Nathan is not author, a contention which has little to no source). In Ha'aretz, an article appeared with this contention, namely the source for the Tu-beshevat custom is Sabbatian or as the headline reads "The New Year for the Trees, Isn't is Shabbati Zvi."
However, a closer look at the history reveals, that although some of the customs on Tu-beshevat can be traced to Hemdat Yamim the actual celebration dates much earlier. Avraham Ya'ari, the noted bibliographer, in an article traced the history of Tu-beshevat. He explains that much of the early mentions of Tu-beshevat were only in the negative, i.e. one can't fast or say tachanun. Obviously, the first mention is in the Mishna in Rosh Hashana which states, according to Bet Hillel, Tu-beshevat is the new year for trees. The new year does not conotate a New Years like celebration, instead, this only has implications for questions of tithing. One can't tithe fruits from one year using a different years fruits. Thus the 15th of Shevat is the cut off point.
Ya'ari, however, notes the first mention in connection to a celbration or the like is in the 16th century. Specifically, R. Issachar ibn Susan (c. 1510-1580) in Ibur Shanim, published in 1578 (the book was published earlier, in 1564, this was done without the knowledge or R. Issachar and according to R. Issachar, with numerous errors) he mentions "the Ashkenazim have the custom [on Tu-beshevat] to eat many fruits in honor of the day." Mention of this custom also appeared in a Jedeo-German Minhagim book first published in 1590. "The custom is to eat many fruits as it is the New Year of the trees."
There might well be the distinction between the eating of many fruits and the formalized seder.
I would say that the non-formalized way of doing it is that, while there are sources for it and kabbalistically-oriented ways of doing it, the modern adoption of this practice is a reform from people who don't find enough meaning in the rituals we do have, and so they seek to innovate their own. As I wrote here:
The second issue is the general tendency to adopt new practices, be they hafrashat challah segulas, saying parshas Hamon during Tuesday of Beshalach, or Tu BeShvat Seders. This is somewhat related to the lack of discrimination mentioned above. The rituals we do have do not have enough significance for us, and so we seek to innovate, or adopt, new rituals.
Sefer Chemdas Yomim from Rabbi Binyomin HaLevi in Tzefas brings down the Seder in the following links