I begin to concentrate on the center section of the design, angling the greenery straight up.
A side view gives you a better angle to see what I mean.
Don't worry too much about each individual stem - you can always trim back excessive length as you go. Just take care not to turn it into a bushy ball!
The bottom stems angle downward, the center stems outward and the center and top stems upward.
The centerpiece quickly takes the shape of what florists refer to as a "long and low" design. This means it fits well on a table, while still allowing people to sit, eat and see each others faces to have a conversation. The chimney and candle don't generally interfere that much.