First of all you need to setup FND%Diag% profile option to yes for the particular user who wants to generate trace.
User can enable trace from any form from Help--> Diagnostics--> Trace (difference options will be there to choose from as per your requirement).
It will display on user screen that actual path of trace along with trace file name.
User has to reproduce issue and once he/she is done reproducing same scenario then you can ask them to turn trace off with the same way they enabled trace. Help ->Diagnostics -> No trace.
Upload trace file to SR/provide it to developer/concern person.
I hope this helps you.