Most importantly, Europa is within a huge radiation belt from Jupiter. The building of radiation deflectors around Europa would currently be highly impractical. A better approach is making an artificial magnetic field to deflect the radiation.
Europa would have to be heated so it can support liquid water on its surface. The problem is that if Europa reaches the shadow of Jupiter, it can freeze again. A few hundred tons of Sulfur Hexaflouride, the most powerful greenhouse gas could keep terraformed Europa warm. The new atmosphere of Europa would have to be composed of 70-79% nitrogen, 19-29% oxygen, and 1% of water, carbon dioxide, hydrogen, argon, methane, ammonia, and a layer sulfur hexaflouride.
After the ocean fully melts, Europa would most likely need artificial continents or floating cities due to the fact the moon has vast amounts of water and a rocky surface would be deep within the ocean. From sunset to sunrise, a night on Europa is about 1.78 Earth-days, so during this time colonists may likely need to retreat underground until the return of day or speed up the rotation.