POGS As explained above the SNP's long stated case has been to abide by the previous referendum result unless Scotland was dragged unwillingly out of the EU and, of course the whole UK is STILL in the EU.
Back in Magaret Thatcher's day it was accepted that all Scotland had to do to declare Independence, was to return a majority of pro-independence MPs to the House of Commons, but this was rapidly forgotten and rubbished when this scenario actually happened at the 2015 GE and again in 2017.
Meantime, the SNP MPs, MSPs and MEPs have had the Scottish Claim of Right (which declares that Scottish Citizens are Sovereign and therefore have the right to determine their own future governance) ratified by the Scottish, Westminster and EU Parliaments. In layman's terms this means that the SG could call a GE where the SNP (and Green?) Manifestoes would include a statement that if they achieved a majority they would secede from the Union, revoking the Treaty of Union, which has now been breached numerous times anyway,
This would be a perfectly legal action under Scottish, Uk and EU law. There has been heated argument as to whether there should be a ratifying referendum to confirm this course of action, but as far as I understand this is not a legal requirement.