How to consecrate a medieval church building

 These rites date from Anglo-Saxon times and consecration was required by the papal legate to England in 1237. 

You are going to need a bishop. He and his entourage should circle the building three times singing prayers. Sand should be sprinkled on the floor in the form of a saltire cross linking the four corners of the church. Inside, the bishop and his staff should write Greek letters along one diagonal and Roman along another, showing that the church is founded on the written Word of God. Sprinkle with holy water. Crosses of chrism oil should be applied in twelve places. Bless the high altar and place relics within it. 

PS. Your church will also need a name, in the form of dedication, which the bishop must approve e.g. The Trinity, Christ, Mary, one or more of the saints or All Saints. Peter, Andrew and Michael would be popular choices, but you might go for a local favourite such as Cuthbert, Botolph or Petroc, depending on where you live, or occasionally the founder. You should probably get a statue of said saint for the high altar. You could also consider a banner. You are going to want to observe the saint's day as a patronal festival. 

Going To Church in Medieval England, Nicholas Orme, p85ff