How the orthodox perform confirmation?

The closest Orthodox approximation to confirmation is "Chrismation", where holy oil (Chrism) is used to annoint the faithful into the Church. After Chrismation, Orthodox Christians may share the Eucharist in the service of Divine Liturgy. You might say the sacrament of Baptism brings people into the Church, and Chrismation brings them to the altar; when children are baptized, they are usually Chrismated the same day. For adults who have been baptized outside the Church, their prior baptisms are accepted if they were Trinitarian (i.e. "In the name of the Father, and of the Son, and of the Holy Spirit"), and they are Chrismated into the Church. Joining the Church is not an event, but the beginning of a process called "Theosis", becoming more and more like God, as we work with Him, working out our salvation with fear and trembling, to be conformed to His will.