probably repeating all you have been told but..

When doing obs write what you see not what you think.. for example if a child is stood crying, write just that, not that you think she is sad, upset .. if one is shouting stamping feet ...do not assume they are angry learn to be factual.

Set a period of time to observe and if possible make sure everyone knows what you are doing as children will always focus on something different and try to distract you, you will find that you are the one they will come to even if others are available. we sometimes have the observer using a clipboard so all staff know what is happening and can divert the children if needed. Get the children used to someone walking around taking notes also helps, eventually they become less conscious of being watched and will play 'normally'.

learn / devise your own version of a shorthand code.. and write up the observation in full as soon as possible after doing it.. surprising what details can be remembered

find an activity to focus on linked to the focus.

we always suggest also ensuring the child is one who is calm, easily watched and at an expected stage of development for the age, don't make it harder than you have to.