On the flight deck there will be a board and its says speed them the next set of flaps how do I know when to have full flaps
I suggest just looking it up online and having it with you. I use my iPad for flying IF, and the mobile phone next to me has all the references I need.
Every aircraft is different.
Depends on aircraft and weight.
You can tell by the attitude of your plane. As you descend, you will want to keep your plane level. As you slow, your nose will start pitching up. When it exceeds 2-3 degrees nose up, you will want to extent your flaps by one setting, to bring the attitude of the plane back to level. You keep repeating this until you are at your full flap setting.
You should be at your selected full flap settings by 1,000 feet above ground, flying level and/or no greater then 2 degrees pitch up.