In The Giver by Lois Lowry, how are the characters acting to create a utopia?
By "the characters" I can only assume you mean the citizens of the community, or perhaps you mean the Elders who are in control of the rules for the community.
The word "utopia" describes a world where everything is perfect. Of course, this is subjective, as everyone's version of "perfect" can be very different. So, we have to examine the ideals of the community to see what it is that they prioritize and we can use that information to figure out what they think is perfect.
The community values uniformity and obedience. Everyone in the community is held to the exact same standards. As far as is possible, everyone looks relatively the same in their dress and in their physical features. We know this because Jonas has light colored eyes and that is considered unique. Everyone else has dark eyes because they have been genetically engineered to look relatively the same. Even the landscape has been engineered to be flat and uniform. So, we know that the community values uniformity. In terms of obedience, everyone is expected to follow very strict rules. They have a curfew each night; they take pills to suppress their sexual desires and must report those desires as soon as they start; they can only work in jobs that are assigned to them by the Elders; they can only marry who is assigned to them by the Elders; they can only have two children, one boy and one girl etc... So, obedience is part of what keeps this community uniform as well.
So, based on all of this we can say that this community values uniformity and obedience and therefore their "utopia" is one in which all members of the society are alike and well behaved. They are accomplishing the creation of this utopia through the rules of their society as well as the science they use to engineer the people and environment around them.