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. 

 The US’ best hope is that a blue state goes first, ideally California as a response to a Trump re-election, albeit I can’t see them doing so.

Because whoever goes first is going to cop the worst of the Federal Government but once one goes, there will be a flurry of secessions in short order capitalising on that sentiment and the Feds won’t be able to contain them all; it’s amazing how quick people can realise the Emperor has no clothes.

I can’t see the US sticking together in this form beyond 10 years.