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Hi. How do I make shaker style bi fold doors for under the stairs. I thought of using 18mm mdf with a 6mm frame on it to make shaker type?

I have used 12mm with 6mm strips to make 18mm thick shaker doors. Weight is an issue with hinging bi fold doors so I would look at a simple framed door with hardboard faces with just the shaker strips I 6mm MDF.

DWD

Note - unlike other members here, I'm a DIYer NOT trade
But i have made Shaker style doors and panels from MDF

i have made a door on normal hinges , and panelling , as DWD said, I used 12mm MDF , but 9mm for the strips, as the other half wanted a deeper step, so that made 21mm - BUT the door and Panel( removable to give access to behind a toilet ) are quite heavy.
I also used Moisture Resistant MDF (expensive now , double the price for a 8x4 sheet) , as i have found that gives a better paint finish , then the normal MDF i used on a Window Seat storage Draw Unit.

I used the 2 part mitre spay glue to fix the strips , which was a bit of a mistake, as getting the exact aligned was a problem on some of the fixed panels - even with a jig, if slightly out, i'm 2mm out on one strip and notice it every time i go into the toilet - so now I use 10min acting glue D4 , which I found out about via a recommended on this forum

Depends on the criteria. Traditionally they would be of mortise and tenon construction. For quickness and economy then dwd's method is far more common and one which I've used myself. The issue or problem can arise with the hinges. Between the two a surface hinge on the inside is the simplest solution. On the hanging sideof the first door I found cranked storm hinges to be a decent option as it avoids having to screw into the edge of the mdf.