Europa is an icy satellite of Jupiter. Terraforming Europa would be far easier than Io, Mercury and Venus, but much harder than Mars. This satellite would first need a new thick atmosphere. The atmospheric pressure would have to be around 7 bars. Most importantly, Europa is within a huge radiation belt from Jupiter. The building of radiation deflectors around Europa would currently be highly impractical. 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 sulfur hexaflouride. After the ocean fully melts, Europa would most likely need artificial continents 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. Another method would be to move Europa's orbit so the tidal heating is stronger and would melt the ice, the oceans and the atmosphere would retain the heat. You would have to add an atmosphere before the ocean is exposed to the vacuum of space and you would also need a magnetosphere, this would be made as a byproduct of the tidal heating as the flexing moon jostled about the magma creating an internal dynamo.