Choose one level of spelling lists and application for starters: A, B, or C. You will want to identify the one level that is the best overall fit for the student and do all of the instruction and practice for that level.
Essentials lessons are designed to reinforce and practice specific spelling words and concepts the students are learning. While the phonograms, spelling rules, and grammar concepts are taught to every level, spelling words and morphemes are added systematically as students progress through one level. Students continue to use and practice them in future units. Because the lessons progress in this way, many activities will not work well or will be confusing if students haven't learned all the previous content from that level.
That said, once you determine the best level to use with a student or class, it is completely appropriate to supplement with spelling words or practice activities from a different level as needed.
For example, an older student who is using Level B for spelling might enjoy learning a few "challenge" words from Level C as an extra spelling challenge. A student doing Level C who is struggling with a particular grammar concept could do the Level B "Identifying Parts of Speech" activity for extra practice with that concept after completing the Level C work. And struggling readers will often benefit from doing additional Spelling Analysis with new words on a different day after learning the list of spelling words from the level they are working on, since this process strengthens foundational skills and helps to build reading fluency. For example, a student doing Level A could do spelling analysis with five to ten of the words from the Level B list as a way to further practice applying the phonograms to new words.
Note that once you have completed a particular level of Essentials through Unit 30, repeating the curriculum from the start at a more advanced level is a great way to review the key concepts for deeper mastery while using them to learn more advanced spelling words and morphemes.