When should I harvest for seeds ?

Hi! Can you advise me on when should plants be harvested for seeds? It's my first grow and I want to get some seeds from a strong plant.

when the buds are done the seeds are done.

u have a separate plant making seeds and u want to know when u can harvest from that plant to get the seeds?

i hope thier not in the same room as the others.

I mean what should the seeds look like to start picking them ? Those commercial seeds are not right-off the tree so I'm asking about the color and other features that indicate seeds are ready to start to stock them

when the flowering stage is done the seeds are done..

A few little leaves popped in the area between stem and branches, but I can't see any "hairs" or other signs of developing her sex. Is it the beginning of flowering period? The plant looks quite strong now. I wonder why the part of the stem that is right above the soil surface is thinner than the upper part? Is it normal situation during growth cycle?