There is a fairly good description of what happens with the sentinel node biopsy on this site and in the My Journey Kit info if you have this.
My experience was the day before surgery I went and had some radioactive solution injected into the area where my tumor was. They then pinpointed the most likely first node that this area drained to by scanning and marked this on my skin for the surgeon to see the next day. The blue dye was injected while I was having my surgery. I had a blue mark on my skin for a long time afterwards, but it has gone now (10 months later).
While you have the drain in I just wore loose singlet tops under shirts that buttoned up at the front. They gave me a bag which the tubing and drain bottle could be placed in and then you just put the bag strap over your shoulder when you are walking around. You really need to wear two piece clothing so the drain can come out at the bottom of your shirt and then be carried in the little bag. Someone makes these bags and donated them to the hospital where I had my surgery. I think this is very common. You can just use a shopping bag if you wish. Hope this helps. Hope your procedure goes well. Deanne xxx