Note that Marty's laundry problem is different from the common laundry problem. What I call "common laundry problem" (the one you seem to refer to) is when you have a big amount of dirty cash and you want it to make it clean in your own bank account.
Now here, Marty's problem is different. He has clean cash of his own, and his mission, to prove his proficiency to Del, is to legally transfer the 8 million to Del's accounts.
I believe the Cartel has various legitimate business activities that include construction materials (we learn later that they do tiles) and play the role of a buffer to launder money. So they need to buy stuff to these companies to clean the money.
WENDY: I've been trying to tell you since yesterday about that half-built house on Sunrise Beach.
MARTY: What about it?
WENDY: Well, I used the money from the Chicago house and I bought it.
Now, I'm no expert, but I'm pretty sure you can inflate construction costs and launder money through it. And quite frankly, I don't give a shit if you like it or not. 'Cause I feel pretty good about it. It's a good idea, and I did it for our family.
Tonight we improvise (S01E04)
That's the point of the church construction idea.
WENDY: So what exactly is their [Snell's] turf?
MARTY: Well, fucking everything, right? Probably why every single business rejected me the second we got in here. Now I got that dead body. No one's gonna wanna work with me. We're screwed. We got $4 million left to launder.
How are we gonna find enough businesses to clean that much without getting into a war with these people?
WENDY: We have to build something of our own.
MARTY: Flipping houses isn't gonna do it, Wendy. Could flip 50 houses, it's not gonna make a dent.
WENDY: Then it has to be bigger.
MARTY: Like what?
WENDY: I don't know. But you can move money through construction costs.
MARTY: Yeah. It'd have to be massive.
WENDY: Yeah! Well, obviously. Lots of people, lots of cash.
MARTY: And no IRS.
WENDY: So what is that?
MARTY: You tell me.
WENDY: There's gotta be some business the IRS is afraid to touch.
Ruling Days (S01E05)
The church builders will buy the needed materials to this buffer company, but the buffer company will write way more materials that were actually bought. For example, if the church builders bought $10000 of materials, the buffer company will write 100000$, and Marty will add $90000 dirty money so thats $90000 cleaned in the process.
He used the same trick to renovate the Blue Cat, when Rachel noticed that they wrote way more materials that actually delivered.
Spoiler if you didn't see S01E09:
When the church was burned down and he wrongly blamed Snell, he said to him that instead of burning, he could have contracted a destruction company (his buffer company) that would have overcharged the destruction and they missed an opportunity to clean more money.