It is easier to place the chicken on a platter or cutting board then run the rod (with the lemon) through the bottom of the chicken and out through the neck. This puts the lemon safely inside. Put the second fork on the rod, pointing towards the chicken and bring it through. It might seem like you don't have enough hands for this operation but be patient. You need to make sure that the whole chicken is tight in the forks before proceeding. Press the forks together, gathering up the legs and wings in the forks so that they are held in place. If you have a dangling wing it will flop around and likely burn.
Once you have the bird secured, tighten the screws on the fork. I suggest you roll it around a little to make sure that the legs and wings hold. Don't worry about the balance, we'll get to that in a minute.
If you look at the picture above you will see that I have pulled the wings through the forks so they can't move. This is how you want to position the chicken.