The surcingle goes on like a saddle, cinched up about the same tightness as a girth. The caveson goes on like a halter, I try and keep two fingers width between the noseband and the cheekbones. It should fit snug so it doesn't slip around when you apply pressure on the line.
I always attach side reins to a bit, not to the caveson itself. I want them adjusted so the horse gets release when he is close to where I want him at that point in training. For young horses, or those new to side reins, I put them fairly low on the surcingle, and adjust them fairly long, to encourage stretching down and forward and lifting the back. For more advanced horses I adjust them up a bit and slightly shorter, but never so tight that the horse is behind the vertical. I try to make it so the horse has release slightly in front of vertical.
I double lunge by using two lines, one attached to each side of the bit. The inside line comes directly to me, like normal, the outside line runs through a ging on the surcingle and either over the horse's back, or behind the haunch, under the tail (which should be tied up for safety). Carry a whip as normal in the driving hand which is also holding the outside rein. It takes a lot of coordination.
I would reccommend getting some hands on help with side rein adjustment and double lunging. Having eyes on the ground can help so much and the difference between correct adjustment and incorrect can be very subtle.
