Ethnocentric missionaries would probably view Igbo culture as inferior to their own, more so depending on how different it is from their home culture. They would encourage the Igbo people to adopt elements of their home culture and drop habits or ways of life that differ greatly from their (the missionary's) culture.

Religion is the most likely element that an ethnocentric missionary would focus on in regards to Igbo culture. They would likely view the religious practices of Igbo people as immoral or misguided at best and blasphemous at worst. They would try to replace the Igbo religion with the religion they are teaching as missionaries. Depending on how much control they had over the Igbo people, they may even restrict Igbo people from practicing their home religion and punish those who do.

An example of ethnocentric missionaries is seen in Things Fall Apart when the Christian missionaries come to Okonkwo's village. At first, the missionaries are treated poorly and don't have as much weight in Igbo political or cultural affairs. But as more Igbo people become sympathetic to them, they begin discouraging traditional Igbo practices and present Christianity as a "superior" alternative. Many, such as Okonkwo, find this loss of tradition distressing and as though the missionaries are dismantling Igbo culture from the inside.

An ethnocentric missionary would be highly critical of the traditional Igbo culture and way of life. The following is an example of an evaluation of the Igbo culture written from the perspective of an ethnocentric missionary:

The Igbo people lack a structured bureaucracy and solely rely on superstitious beliefs to guide their village. For example, a group of masked men called egwugwu represent the village's ancestral spirits and are revered judges of Umuofia, which is quite ridiculous and unsound. Their culture is also founded on violence and bloodshed. The entire village gathers to witness a wrestling tournament and the men are given titles for bringing back the heads of their slaughtered enemies. They also lack a firm understanding of God and are polytheistic. The majority of their gods are depicted as selfish, greedy deities, who they fear and must appease in order to have a successful harvest.

Their suspicious nature is also exhibited by their belief in ogbanje, who are wicked children that die and return to plague their mother's womb. The uncivilized Natives also believe that twins are wicked and cast cursed people into the Evil Forest, which seems to be a perfectly fine plot of land to build our church. The Igbo villagers also worship and revere the Oracle of the Hills and the Caves and are willing to kill innocent people upon the oracle's request, which is what happened to Okonkwo's surrogate son Ikemefuna. Men are also allowed to marry multiple wives and spend the majority of their leisure time in idle chatter while the women take care of the compound. Overall, the Igbo culture is completely backward and the uncivilized villagers lack a fundamental understanding of religion, language, education, and bureaucracy.

Before you write, it is worth reflecting on the relation between your work and the text you are studying. One of the primary themes of Things Fall Apart is the richness and dignity of the Igbo culture, the stories and ceremonies of which are described in detail. Achebe is writing in response to and against the type of document you will be composing. The greatest limiting factor for your writing will be the lack of those cultural details which abound in Things Fall Apart, but which an ethnocentric missionary would not know and would not attempt to find out. The tone might be something like this:

The Igbo tribes of Nigeria are a primitive people, living primarily by subsistence agriculture and fighting frequently amongst themselves. They practice a polytheistic religion based around the worship of wooden idols. Since a church has been established on the periphery of one of their villages, a small congregation has grown up, but they have been very slow to grasp even the most basic tenets of Christianity. We are now in the process of translating the Bible into Igbo, but since very few of them can read or write even their own language, this endeavor cannot be expected to bear fruit immediately.

The culture of the Igbo people is basic and brutal, often centered around ritual slaughter and large public exhibitions of gluttony and drunkenness. Their music is mainly percussive, using the same drums that they employ for communication between villages. They show little aptitude for the visual arts except in the creation of colorful but crudely-executed masks which they use in their rituals. Lacking the cultural infrastructure to make rational use of their leisure time, they spend an inordinate amount of this in idle gossip and in quarreling with their neighbors.

In the voice of an ethnocentric missionary (not my own, just want to make that clear...)

The Igbo are an extremely backwards society, worshipping a number of gods rather than the true god we know.  They are focused on appeasing the wrath of these gods and believe that certain people are outcasts and cannot be part of their society.  They even believe that twins are evil and should be left in the woods to die.  They believe that the spirit of one person can come back over and over in the body of a man's children like Okonkwo's daughter.

They don't speak English or any other civilized language and lack any of the things that a modern civilization would require such as a system of laws or justice.