Your International student office should have all the information. This is my understanding:

your ISO officer will issue a new I-20 with OPT information and this info will also be fed into SEVIS system then after that you fill out the extension form and include all the info asked along with the fees. mail it to your USCIS location

make sure you do this before your current OPT expires!

if you have filed for H1 and OPT expires before oct 1st, you don't have to do anything it is automatically extended until october 1st. However keep in mind that if H1 gets denied you won't be able to get that 17 month extension. so plan ahead.