Step-by-Step Guide:
Creatives should be trafficked as Standard 3rd Party Creatives (not AMPHTML or Custom Creatives as per Google's Suggestion)
(i) Generate New Creative:
(ii) Associate New Creative with an Advertiser:
(iii) Select Third Party Standard Creatives:
(iv) Name and set Target Ad Unit Size to 320x480 or 320x400:
Format dependent - all formats work in a 320x480 though:
(v) Code Type should be set to Standard (NOT AMP):
(vi) Copy & Paste Script (typically found in column J of the excel file provided) into the Text Box:
Potentially you may need to replace
ord=[timestamp]
with ord=${CACHEBUSTER}
(Article on more detail into this change here)