You've got a couple options for tying down.
First you can tack a piece of string to one wall about eight inches above the soil, which will support the base of the plant while the second piece of string toward the top of the plant pulls it into a horizontal position. You dont want to go too tight to begin with, but for the most part she's going to be pretty flexible. Just keep tightening the string every day or two until you think you're lower branches are getting the most from the light. You'll see everything start to correct their angle of growth within a couple hours from tying down.
Your other option, either used along with the above or on its own, is to tie down those lower branches using drilled holes around the rim of your pot and paperclips or string. When I go with this method it's usually with a shorter, bushier plant, but you can definitely utilize it with the longer branches toward the bottom of your plant. Also, when I go this method, I try to tie things down in a spiral pattern. It helps keep everything on a horizontal path.
I dropped some pics below, though they're not perfect, of the paper clip method.