After orthodontics and filling remaining gaps, how would you safely do retainers to avoid teeth moving?

Hi, I'll wear braces for 9 months (up an down) . My lower teeth are too large and my upper ones aren't large inoff. To correct my lower croocked incisors, there will be some streeping done to make them look straight. For the upper teeth she'll move the gap I have between the 2 central incisors on the sides of each lateral incisors and close remaining gaps a with composite.. What about the retainer (thermo formed for night + fixed ones), when would have they to be made and put on ? Thanks