Well, you essentially have three main types of lasers that you can do. there high and low SHL, and then there's IJDL. Marth can resond to all three in different ways. Let's start with the most obvious point.
Of course, IJDL is most effective as an anti aerial, so you should use it when you think he's going to try to jump over your lower lasers. Note that this should be in anticipation of his jump, not in reaction to it. For example, at ~10 seconds into the first match, the marth double jumps, and then afterward , you do an IDJL. This didn't serve any purpose, since he was already back on the ground, and it left you pretty vunerable. The idea is that you should try to guess when he's going to try to jump, and then snipe him with IDJL. If you can do this even 25% of the time when he jumps into the air, it will really discourage him from leaving the ground, which is good because it means that you can pester him even harder with low lasers.
Now, when marth is on the ground, he has basically two ways that he can maneuver around your lasers. First, if you shoot a high laser, he can crouch/dash under it. Second, if you shoot a low laser, he can short hop over it. The short hop requires more commitment from the marth, and is less dangerous to you, so you should probably make your default to do a low laser. This prevents him from being able to dash around a lot, which is something that got you grabbed several times in your matches.
However, make sure to pay attention to what the marth is doing, and switch up your lasers as necessary. This is really the key. If you get nothing else out of this, then get this: the type of lasers that you should be decided in reaction to what your opponent does to deal with them.
The last thing that marth can do to deal with lasers is to shield them, and then do stuff like wavedash or aerial oos to close the gap. This is where spacing comes into play in a dramatic way. Consider what happens when marth shields a laser at various distances.
If you are far away, then marth has a lot of options. He can safely wavedash oos here, and close the gap, or retreat even further. He can also try to jump over the next laser, which you should know how to handle by now. At this point though, your lasers aren't doing very much in the way of pressure. They are, at most, a slight inconvenience to the marth.
Now consider what happens when the marth is shielding close to you. If you are right on top of him, then you can go into shield pressure mode, which puts you at an advantage if you're smart about it. However, if you are a just little bit away from him, then he can aerial oos or grab you, and then you're toast. So what you've got to if you're going to approach him is get past his **** zone safely.
Now, if you go just a little bit further away, about the distance of one full wavedash, then you are in a really magical position. At this distance, if he shields a laser, he has no safe offensive options. You are too far for him to be able to grab or aerial oos, but close enough that you can react and hit him if he wavedashes or full hops. So the only thing he can do to is roll or wavesdash back. If he rolls toward you, you should be able to **** him. So he has to retreat. If you can chase him while maintaining this perfect spacing, then that's really good.
tldr: laser him in such a way that minimizes his movement options, then punish whatever he has left.