You can install DCE with or without composer.
Just require DCE on cli:
$ composer req t3/dce:"^2.7"
When composer is done you need to enable the extension in the extension manager.
Caution, when you also use EXT:gridelements in your TYPO3 project!
It can happen that DCE content elements have no contents available in Fluid template, when they are located inside a grid column. This affects also all plugins, using pi_flexform.
When Gridelements is installed, it applies “magic” to pi_flexform by default. In order to make TYPO3 work as expected, you need to disable this behaviour on your own, using TypoScript like this:
tt_content.gridelements_pi1.dataProcessing.10.default.options.resolveChildFlexFormData = 0