Here's my advice:
1) Take a lot of pictures, specifically of important moments of the battle and *especially* of deployment. These will help both in making the battle easy to follow and also in reminding you of what happened (since the pictures will be in order on your device and you can scroll through them as a kind of reminder).
2) Feel free to take notes. My memory is pretty solid, so I don't usually. If you get a couple small details wrong here or there, not only will nobody fault you, but also nobody will care that much. So long as the main details are clear, the battle will come across fine.
3) I advocate using a map program of some kind. I use Battle Chronicler, which is a bit old (last updated in 2011), but perfectly functional and generally user friendly, once you get it set up. Maps make things vastly clearer to the reader, though they can be time-consuming to create.
4) I organize my bat-reps thusly: I often open with a dramatic vignette (totally optional, by the way--don't feel obligated), then I present the two army lists, then I go over the mission, terrain, and deployment, then I cover each battle round (turn by turn but *not* phase-by-phase), then tally the final score, then present a brief post-mortem discussion of how I felt the battle went. I feel this is the clearest and most thorough way to present a battle without getting bogged down in minutiae.
5) When describing the action of the turn, it isn't important to present that action in the order in which it occurs most of the time. In other words, don't tell us the whole movement phase, then psychic phase, then shooting phase. This is confusing because the reader's mind has to keep bouncing from unit to unit across the board. Instead, describe what a single unit or whole group of units do for the entire turn all at once (e.g. "The tactical marines rapid-fired the guardians and then charged them, wiping them out. Meanwhile, on the other side of the battle, the dreadnought got its butt kicked by the Avatar). Doing it this way makes the battle more narrative and therefore easier to follow.
6) Describe direction as North/South/East/West. Do this because it makes direction *objective* to the map rather than *subjective* to the viewer (ie "My right" changes according to where you're standing, where as "east" is fixed according to how the board is oriented).
7) Whenever possible, lump descriptions of units' behaviors (or even entire armies) together. Saying "my whole army moved south towards the Tau gunline" or "all the boyz advanced and shot their shootas at the closest targets they could see." This makes it easier to understand what is going on for the same reasons I explained in #5.
8 ) Don't use abbreviations for anything. Saying "my CS used their 2x BC to shoot the SBs by the Obj" is much harder to understand than saying "My crisis suits shot the shoota boyz by the objective with their double burst cannons." I know this means it takes longer, but the extra labor on your part will result in greater clarity for your reader.
9) Obey the laws of grammar, syntax, and paragraph organization wherever possible.
10) If you do include a dramatic vignette/multiple vignettes, try to keep them short. Nothing kills the momentum of a bat rep faster than having to read four pages of somebody's prose mid-turn 3. I include mine at the beginning only. Sometimes I'll do beginning and end. If I'm doing a very narrative-style batrep, I might do a very short one after each battle round. But SHORT is the key, here. Less than 250 words, if possible. No more than 500/750-ish.
I probably have other things I could say, but those are the essentials.