CMO installation via deployment center is the "best practice," but it only supports sites on 7.5 (before nuget updates). So if your site is already on 8, you'll need to install CMO manually. We don't have an official how-to guide on how to do it, but here are some general resources you can look at it to help you install CMO manually (in-line notes from CMO dev):

Installing CMO on website consists in copying binaries and registering CMO module, location and handlers/modules in web.config.
In order to see what should be added in web.config, You can create plain Alloy site, backup web.config file, install CMO using deployment center and compare updated web.config with original file in any merge/comparison tool.

There some information pieces that you can use to install/uninstall CMO manually:

Installing and configuring the aggregation service (written for previous CMO versions, but should be accurate):
http://world.episerver.com/documentation/Items/Developers-Guide/EPiServer-CMS/75/EPiServer-CMO/Installing-and-configuring-the-aggregation-service/

Configuring the Statistics Handler (written for CMO 2.0, but in general is accurate. I think browser definitions are not required now.):
http://world.episerver.com/documentation/Items/Installation-Instructions/EPiServer-CMO/CMO-20/Configuring-the-Statistics-Handler-in-EPiServer-CMO-20/

Configuring after the Installation (written for CMO 2.0, but most of the steps are accurate):
http://world.episerver.com/Documentation/Items/Installation-Instructions/EPiServer-CMO/CMO-20/Installation-Instructions---EPiServer-CMO-20-on-EPiServer-CMS-60/#postinstall

Uninstalling CMO (written for CMO 2.0, but most of the steps are accurate):
http://world.episerver.com/documentation/Items/Installation-Instructions/EPiServer-CMO/CMO-2-R2/Installation-Instructions---EPiServer-CMO-2-R2/#uninstall

Configuring EPiServer CMO Thumbnail Service on Windows Service 2008 R2 or later versions
http://world.episerver.com/Documentation/Items/Installation-Instructions/EPiServer-CMO/CMO-20/Configuring-the-Thumbnail-Service-on-Windows-Service-2008-R2/

Basic CMO component and deployment scenarios:
EPiServer CMO components and simple installation scenario - http://dmytroduk.com/techblog/episerver-cmo-components-and-simple-installation-scenario
EPiServer CMO: advanced deployment options - http://dmytroduk.com/techblog/episerver-cmo-advanced-deployment-options
EPiServer CMO in load balancing environment: simple scenario - http://dmytroduk.com/techblog/episerver-cmo-in-load-balancing-environment-simple-scenario
EPiServer CMO in load balancing environment with separate editing server - http://dmytroduk.com/techblog/episerver-cmo-in-load-balancing-environment-with-separate-editing-server

Once you get CMO installed on 8, you can then update CMO from NuGet.

Thanks.  I'll give this a shot.