I Have Been Taking Effexor 75 Mg Bid For Many

I have been taking Effexor 75 mg bid for many years. Recently, I ran out and was off my meds for 2 miserable weeks. A friend/prescriber gave me samples of Citalopram which I have taken for 2 weeks at 20 mg once per day. I now have the chance to go back on the Effexor - how should I proceed with titration and such?