The longer you do the newbie routine before adding the extender the better. Even if it’s just to get to know your penis better and how it is behaving and responding now you are doing PE.
Try and read up as much as you can on the forum to find out what you can about extender use. When you do think you are ready and you are starting early in your PE ‘career’, be very careful and use it very lightly and build up slowly the duration and intensity of use. If in any doubt, don’t use.
I don’t think that anyone will give you a 100% certain answer to use it or just do the newbie routine. Each guy is different and some will be ready to use it and some can only do a light newbie routine. If you are doing a full newbie routine and you have gains why change. If you have no problems but no gains after a month or 2 (or better 3) then maybe you would think about adding it. If the newbie routine is too stressful then do not add.
I hope that wasn’t too rubbish advice. Basically, be careful whatever you do. Good luck!