20th Century anarchism, the majority of it anyway, drew principally on one main source for its ideas about organising. That source, surprisingly, was not any of its most famous adherents. As a rule whilst the likes of Kropotkin, Emma Goldman, Mikhail Bakhunin, Pierre-Joseph Proudhon had a lot to say about how society was at the time, and how they wanted it to be, they didn't have very much specific to say about how an actual anarchist organisation should be. There's no Lenin or Trotsky to go back to and say: "this is the role of the party", "this is how we should make decisions", "this is what a united front is for" "this is how we act in the trade unions".
No, when anarchists organised the ideas on how to do it came out of working class experience. The most significant things that anarchists have ever done came not out of applying works of theory, but on generalising activities that were already happening and developing them. The main current of 20th Century anarchism was anarcho-syndicalism. Anarcho-syndicalism as a form grew out of French trade unionism at the turn of the century. In an earlier life modern Stalinist monolith the CGT (Confederation Generale du Travail) was the model for most of last centuries anarchism. (If you want to know more about the history read this article I wrote a long time ago)
The ideas behind syndicalism were simple. Form a trade union that mirrored the structure of the society that you wanted to create. The early version CGT had several main features:
1 - It was industrial and regional - unlike earlier trade unions which organised workers based on what they did, the syndicates organised them on where they worked and in what industry they were part of. The Bourses de Travail (Labour Offices) then organised them regionally.
2. They were federal and decentralised - Modern trade unions elect a permanent leadership to make formal policy and decide on union strategy. After creating the network outlined above, the CGT made sure that every group was autonomous, controlling their own funds, and that what formal policy that existed came from the bottom upwards.
3. They had mandated delegates and direct democracy - to overcome the difficulty of operating a direct democracy among half a million workers, the CGT invented a system of mandated delegates, in which workplace groups elected people to represent them in regional, industrial and national forums. Crucially, they didn't just elect people, they discussed and agreed upon (in advance) the decisions they were going to take and the things they would argue for. Finally they had the right to withdraw a delegate at any time, if they strayed from their mandate.
4. They were unaffiliated - despite electing a succession of anarchist General Secretaries, the CGT did not have a specific ideology, and did not support a political party. Each political current was free to organise within their ranks, and argue their particular ideals and policy. Directly supporting electoral candidates and political parties was outlawed in their constitutions. This was an organisation for the entire working class.
5. They made decisions - although the CGT was a pluralist, de-centralised organisation of hundreds of thousands of people, it had a functioning decision-making structure. At a workplace level it was able to agree through majority voting on the policy that individual syndicates wanted to follow. Through the system of mandated delegates they were able to create a coherent national policy that derived from the base of the organisation and could not function outside of their wishes. It wasn't a case of everybody wander off and do whatever they like, but of an organised democratic system of solidarity and mutual aid. People were free to hold whatever political views they liked, and knew they had a forum to express those views, both in their workplace and higher up the chain.
These groups used democratic and anarchist methods of organising to empower all of the members to form and act on their needs and desires. They used robust forms of decision making because their working lives were often precarious (job security? living wage? Pah!) and their class struggle brutal (kettling? Try live fire and saber charges!).
These forms of organising, taken directly from what workers did anyway, were adopted in various forms across Europe (famously in the Spanish CNT). I mention them now, not because I think they should be held up as a model for anyone to follow, (they grew up, as any form does, in particular historical circumstances) but because they outline what hard-headed organisers our anarchist predecessors were. They didn't let anything and everything go so as not to tread on anyone's toes, they made an organisation that was fit for purpose. It was non-sectarian because it's basis for unity was strong. It was autonomous and anarchist because it gave its component parts the ability to take their own decisions and direct their own resources. It was democratic because it had adequate structures for people to express their democratic opinions in an open forum. It was organised because it had mechanisms to make decisions and then stand up for them.
It was an organisation that had a clear idea of how to organise people and what for. Which is something that all organisations eventually have to do. You have to decide: is your group a social movement? Is it something aimed at organising a particular constituency and expressing as directly as possible their needs and desires? Does it have the structures to do this? Is it fit for this purpose? Or is a publicity campaign aimed at spreading a certain message? If so, is the message it spreads a coherent one? How do you come to decide what that message is and continue to hold your adherents together?
20th Century Anarchist organising was about the abolition of hierarchy and authority, it was not about the abolition of organisation, coherence or formal decision-making. Anarchy, as they said, was the highest form of organisation. It never meant, nor should it mean now, a complete free-for-all.