Contrary to popular beliefs, anarchists have no problem with constitutions or laws or governments. Anarchy is ANti-hierARCHY, which in the extreme case extends to not accepting the hierarchy of the State (i.e. an organization with a license to exert force over all others in society). But,

1. Many anarchist movements do not demand this much change. They only demand removal of specific forms of hierarchies they think most problematic. The Occupy movement for instance was demanding the curtailment of the political power of the 1% over the 99%. Its always better to think of political movements as directions of evolution in political space, rather than specific destinations.

2. Then how do anarchists propose that laws/constitutions be imposed? By consensus and discussion. By making sure everyone is on board. Or by temporarily giving someone conflict resolving power (as in the Debian case). Plenty of societies and organizations operate this way, and work fine. Read The Dawn of Everything for some historical examples. See a region in Syria [1] as a modern example.

[1] https://en.wikipedia.org/wiki/Autonomous_Administration_of_N...