Slashing is an art form! Harder to get right on highly hydrated doughs like the no-knead types (but you can use kitchen shears).
Slashing TOTALLY changes the shape of the loaf.
slash in a criss-cross pattern or as a square, and even the longest batard will attempt to rise as a round boule. If you have a round boule it will rise evenly.
Cut in straight lines and it will widen against them, so if your loaf is round and you cut it as ||| -it will square off and become wide and long. If your loaf is long and you slash it as ||| it will widen and shorten. If your loaf is long and you slash it as ===, it will lengthen and turn narrower. If however you slash it diagonally //// , your loaf will rise at the same proportions as it looks in front of you. Experienced bakers do a combination of cuts in different directions that would result in a final desired shape balance. I think that you can get the hang of it by doing about 10 loaves or so.
Slashing isn't just pretty and good for the shape; it provide a weak point in the crust that breaches as it rises, so instead of letting the loaf decide on its own where it will break up and tear a "wild fissure", you are deciding where this will happen by providing the weak points. If you don't do it, the fissure will almos always defelop where you don't want it, across the bottom of one of the sides. It's annoying...
To prepare for a good slash, you need to practice some loaf forming where the most stressed and tensioned area is the top of the loaf. Also over-rising it before baking will make an ineffective slash and oven spring.
I use a lame to slash but in softer loaves I find that an serrated meat carving knife works well. You can also do good things with shears if you practice them. Hope this helps.