I noticed small amounts of blood on her bedding as first sign (also never had a dog on season, so wasn't sure if I missed any other signs)... this increased slightly, but was never a massive amount, she basically kept it clean.
I walk her off lead and for the first ?week, we were fine, but then the dogs were starting to show interest, and she became quite aggressive towards other dogs, snapping at them (ie. being a bitch).
So we kept to on lead walks and parks with no other dogs etc. Her privates became very enlarged, and she developed a certain swagger as if to waft it about. I think once the bleeding has stopped (and the swagger starts) that is the fertile period for about a week, and then another week for everything to settle.
I think I kept her isolated for a good 4 weeks from first signs, just to be sure. And then I had a freak out, as her nipples became enlarged and I felt a bulge in her tummy afterwards - not sure if it was a phantom pg, but it was nothing and everything settled within another 2 weeks.
We then had her spayed before she came into season again... only downside has been she has zero tolerance of dogs trying to mount her, which whilst understandable, her reaction goes from zero to growling/snapping very quickly. Before her season, she wouldn't react to this at all.