I have not yet planned my cover art.
As for marketing - that is still a work in progress, and it will differ for each book.
The books that I am working on are technical ones: one group of them is oriented toward a particular microcontroller, while the other group of books is oriented toward a particular hobby.
In both cases my plans include first developing web sites and collateral material (including videos and perhaps even online courses via Udemy and occasional e-newsletters) with the goal of building an audience that will be tuned in to the same subject area as the books - if I am successful in rolling out that strategy then I ought to have ready-made audiences to market the books to. In one case I'm also developing an electronic product that is complementary to one of the groups of books, and so there will be opportunities for cross-promotion. I am also open to finding partnership opportunities with vendors and influencers in those fields (who do not compete with my products but rather may be complementary to them), with the goal of getting access to their existing audiences.
While I haven't done this before by myself, I have been involved in doing such things for others, and I know how to do all of it by myself, from creating the web sites to handling publicity and everything else surrounding it.