The Uncommonwealth achievement requires the player to start as 立陶宛 and form the Polish-Lithuanian 波兰立陶宛联邦. The Commonwealth may be formed though a number of decisions, see Commonwealth#Formation.
Form Polish-Lithuanian Commonwealth (Diplomatically)
波兰 begins the game in interregnum. So simply royal marrying Poland is usually sufficient to ensure that the same dynasty gets on the throne, barring the exceptional royal marriage with 法兰西. While waiting for the interregnum to end, the player should be seeking alliances with Poland's rivals, who hopefully also border them, such as the 条顿骑士团, 波西米亚 and 匈牙利, as well as making sure to take every possible event option to increase prestige, as this will need to be higher than Poland's in order to claim their throne. Assuming the interregnum ends with the same dynasty on the Polish throne, one strategy is to immediately claim the throne, and call as many of your allies which are also Poland's rivals into the war, on the promise of land. If successful in the war, it is advisable to not keep the promise to grant land, as this would diminish the Poland over which you will be forming a personal union. This admittedly comes with the downside of a significant trust reduction with those nations; however, with Poland under a personal union, and all of Poland's vassals under them, other alliances are less necessary and the player is free to seek out new alliances. From this point, once tech 10 is reached, the Decision to form the 波兰立陶宛联邦 diplomatically should be available.
Enforce Polish-Lithuanian Commonwealth
Alternatively it is also possible to 'Enforce Polish-Lithuanian Commonwealth'. This will involve conquering many of the Polish provinces as cores on Danzig, Warszawa, Krakow, Vilna, Minsk, Kiev, Turov and Marienburg are required. Make sure to not release Mazovia from Poland during these wars, or else they too will have to be conquered, as Mazovia must not exist in order to use the decision to form the Commonwealth in this way.