In what ways is it the best to use a computer in correspondence chess? I have only recently started correspondence chess and I have 
read that engines are allowed in correspondence chess. I definitely do know that the world champions of correspondence chess have said that using engines to a 100% degree is a bad idea.  My question is this: What is the most efficient way to use engines in this format of chess? Any experiences would be helpful!
Obviously, as long as they are permitted within the rules of the organization, here is how I would use a computer.

First, realize that different programs are better at different things, and I will divide them into two categories. I heard GMs Peter Svidler and Jan Gustafsson discuss this during the Firouzja-Carlsen game just yesterday.

The first program is for opening play, and that is where Leela shines. I have not had much experience with this program, but Jan mentioned that it can, apparently, blunder at times. They both agreed that Stockfish is still the king of analyzing in the middlegame. I watch a lot of relays of top tournaments, and I follow along with Stockfish, and it is currently the best program out there for the middlegame, and the best part is that it is free.

The next thing is that in most circumstances, I only look at the top two lines (in ChessBase, you can click the little plus/minus sign in the engine box to add/remove the number of lines). I would probably increase this to the top 5 lines for correspondence since there may be a line that fits in better with what you are trying to do, so you can, at least, know to explore it.

Next, in the opening, it helps if you have a good understanding of the opening pawn structure, and the typical plans that are played. As you get into the opening, look at a bunch of GM games from a database to see what they have done before you decide on your moves. Have Stockfish going while looking at them...you never know what you might find if it pops up an unusual eval.

Lastly, and the stronger you are, the easier this is, and the better you will be at it, but you want to play "advanced chess". "Advanced chess" is term for using both the computer and your knowledge to pick the best move. In its infancy, Anand was considered the best at this, and there were a few tournaments where these top players were allowed to use a computer. Here is an article. As part of this cyborg play, and this is the key, you want to force in certain series of the computer's best moves or even some of your own human moves, so, in essence, the program only starts analyzing from that point.

For example, if you and I are playing and we are on move 28, and you just let your computer analyze from there, it may analyze out to move 38. Not bad, but if I am able to force in 7 likely moves based on my 40 years' of experience and the computer's evaluation along the way, and only then let it think uninterrupted, I start at move 35, and might get to move 45.

If you are evaluating a position based on the end result at move 38, but I am evaluating it based on move 45, 7 moves later; I am much more likely to be successful, and win the game.  

That is how I would use one if I played correspondence chess. Frankly, I am too impatient so I was never into correspondence in the old days of postcards, but the advent of computers really turned it into more machine versus machine, so that really killed any chance I would get into this in the future; but to each his own.