Use this guide to upgrade the RAM in your 2017/2018 model 27-inch iMac Pro up to a maximum of 128GB (32GB x4).
Note, the below video details the procedure for opening up the 2017 21.5" model iMac, and the procedure is very similar for the Late 2017 27" iMac Pro. For more details, also see the iFixit teardown guide further below:
Note, the below guide is a preliminary teardown of the 27" iMac Pro Late 2017. Although it is not the full guide, it should allow most competent users to successfully upgrade the RAM. See the video guide above for details on the 2017 21.5" model iMac RAM upgrade procedure, which is very similar machine.