No offense, but the issue is pointless. It is like asking "how can one legally murder someone" because Taiwan has already fxxked up ROC's constitution. If people sitck to the law, there is no way Taiwan can break away from mainland according to ROC constitution. See explaination further down.
Here is the constitution of ROC at its first edition 1946.
or
Constitution amendment can only by passed by National congress, either proposed by Legislative Yuan or National congress itself.
Here is the amendment up to now
Amendment (including territory change) must be proposed by 1/4 Legislation Yuan, 3/4 attending, and 3/4 of the attending deputies' approval to become a referendum to be voted on by people in "free" area of ROC. The amemdment is passed with 1/2 of the voting population. Article 174 of constituion does not apply.
This is a self coup d'etat. The amendment invalidates article 174 which needs a vote from National congress whichs in turn needs electors from mainland in the first place. It also deprievs mainlanders constutional rights without going through the amendment procedure which is article 174. To put in a simple term, two people signed a contract which states "contract terms can not be alterred without consent from both parties". However one guy later declared to invalidate this clause giving him sole "power" to invalidate this clause. What is that!
So if we speak purely about law and logics, All Taiwan's amendments to the constituion so far are against ROC constitution. Any future action to change ROC territory without voters from mainland (to form new session of National Congress) is against constitution of ROC.
Technically a loophole. Those additional articles are actually not part of the constitution and specifically specify that they only apply in the situation before national unification. That would mean any territory change voted by the “free area” without the National Assembly would become invalid. In a way this makes sense, because if national unification occurred, then naturally the territory change would be invalidated, lol.
Why do people expect a Taiwanese declaration of independence to follow some kind of legal process? Did Mao follow ROC law when he proclaimed the PRC? Of course not. American independence didn't require a referendum. At the end of WW1, the end of the German monarchy was also just proclaimed, illegally, but everyone followed
When a new state is started by proclamation it only needs one guy to shout something, no vote is needed. If everyone follows, then that's enough. The precedent is very clear
So of course a DPP president could proclaim a republic of Taiwan and announce the dissolution of the ROC and then build a new assembly for writing a constitution for the new country.
The only reason why they might organise a referendum on independence first is to get international legitimacy and military aid. But again, they could claim that the next presidential election is essentially a vote on independence and then use a landslide victory for justification of a declaration of independence
Of course it is unlikely to follow a long drawn out legal process, but the problem with such an illegal declaration is that you risk losing your own support within the territory. This is the situation that happened in Ukraine 2014.
The situation in Taiwan is not like that of PRC where the majority of the support was behind the Communists. The majority of Taiwanese do not want war. The DPP did not even win a majority of votes. Tally up KMT, TPP, and non-voters, that is the majority. I am in the belief that should there be an illegal declaration, I think there would be a military coup.