creating XML nodes using Nokogiri

I am having problems creating nodes and adding them to an XML file

<mainnode>
    <secnode>
         <data1></data2>
         <data2></data2>
    </secnode>
</mainnode>

i want to be able to add to the file like so

<mainnode>
    <secnode>
         <data1></data2>
         <data2></data2>
    </secnode>
    <secnode>
         <data1></data2>
         <data2></data2>
    </secnode>
</mainnode>

I am having trouble getting the concept of adding nodes with Nokogiri

current code:

def openXML
    f = File.open("file.xml")
    doc = Nokogiri::XML(f)
end

def parseXML
    mainnode.name = 'mainnode'
    f = openXML
    temp = Nokogiri::XML::Node.new "secnode", f
    mainnode.add_next_sibling(temp)

end

What concepts am i missing?

i need to be able to add instance variables to and but i am finding the nokogiri tutorial to not be of much help in this area and have not made it past just adding the node as a child of

any help is appreciated. comment if more info is needed