Hi.
First you will need to acquire a Ethernet wire. Secondly you will need to insert the ethernet wire into the back ethernet port of your Netgear WNDR3400 and the same wire into the back of your Apple Airport Express.
Finally configure your Netgear router and you should be good to go.
(note if the old Netgear WNDR3400 was from your ISP (Internet Service Provider) this may not work)
Thanks.