It will probably have to happen in a way that is independent from the courts and formal declarations. Once there is enough popular support in Texas it’s possible that non-compliance with Federal laws and demands can happen at a large enough scale that it might as well be secession. Feds will respond with troop deployments, sanctions and aid cut-offs, but as long as Texans remain peaceful (at least at first), it can be made to look like a really bad (for the feds) political knee-jerk that snowballs independence movements nation-wide.