I don't mean to be rude, but this question has been answered about eleventy brazillian times. Did you try a search? You did but nothing matched you say?

Okay, here is a quick tutorial.
You need to place your layouts in the layouts folder in mamewah. All in the same folder. You should rename them whatever you like, but a good bit of advice would be to call them by their emulators names. i.e. mame.lay, mame.main nes.lay, nes.main (I can't remember what the jpeg extension is, I believe it is .main, maybe not, but that should be obvious if you look at one of the layouts you downloaded.
So once you have your layouts(.lay) and backgrounds(.main) together in the same folder in the layout folder, use the layout editor in the mamewah folder. Go to open, find the folder they are in, open up a .lay, next, under one of the other menu tabs at the top, select background image and find the matching .main. Once the background shows up, you can edit font sizes of do a lot of other things. Make sure you save the results and viola, you have a layout ready to go. Make sure you tell the mamewah.cfg or mamewah.ini(I can't remember which, but it should be obvious) where to find the layouts at. Hopefully I just saved you five minutes of research.

Also, next time, RTFM. Thank you, who's next?!