You will need to log into your IDT web account using your Login ID and Password. Once you go to the Order tab, you can select from a menu of products. For custom oligos, you will need to provide a sequence name and sequence, as well as select a synthesis scale and purification type. Click on Add to Order when you are ready to add your items into your shipping cart. Upon checking out, you will need to verify your shipping an dbilling address and provide payment information.
There is a guide to ordering on the General Information section of the Catalog.