Hello,
I can't help you with the guitar, since I have not made one myself. My cousin wants one next month and I plan on making a template to use when I carve the cake.
MMF stands for Marshmallow fondant. You should be able to find quite a few recipes on this site for the fondant.
Good luck with your guitar.
I've done a life-sized one before, and all I did was trace the actual guitar to use as a template, then cut it out of sheet cakes and pieced it together where I had to. Also for strings, I used thin, dry spagetti painted with silver luster dust laid end to end. Alot easier than trying to pipe a straight line all the way up the neck. Hope this helps!
Well idk about the taking back part { poor girl }Girl keep busy the more busy u are the better but ,as for cake I made my papa this guitar cake it was a quicky he reqested some runny cream cheese frostin with pecans and raisens so the decorating part is not best for this but u know the old fashioned cakes are harder to deco . Alright this was made with a cookie sheet 3 layers I think I used two 4 the base and one stacked and cut for the top part and the I froze it ,, pulled it out and basically drew the shape with a push pin and just went to carving but hun it has to be frozen { wrap it in foli and seran } Ok next brush off crumbs and crumb coat I recommend if ur a newbie on decorating a carved cake the sides will give u trouble { so just use a star tip 21 or # 32 on the sides and just deco the top it will much easier . But this cake was simple ,
Mashmallow fondant I have a picture of a green Ben 10 cake on here that has it . I'ts simple to make and taste good [ THE KIDS ALWAYS EAT IT } But i'd stick to bc on the guitar cake to many curves . Agian I am a newbie so do as u wish hun lol
MMF stands for marshmallow fondant.There are several types of recipes listed on this site that are pretty simple to make.As for the guitar cake question,if you are making an acustic guitar,you can bake 2 large round cakes to use for the body and cut out the center of each cake for the neck,or if you didn't want to cut the cake you could bake a square one and cut to size.If you are wanting to make an electric guitar I would suggest baking a sheet cake and cutting it to the style you want.As for the strings I still have to pipe them on but I wish I could come up with a better solution,I still cannot pipe very straight.I have a guitar in my pictures but it is the wilton shaped pan.It only holds one cake mix.Hope this helps. 
I have done a scaled one, I get the guitar shape from google, and and print it.
Using from the print out I made the neck first from gumpaste/fondant a week before baking the cake, to make sure it strong and dry properly.
Bake rectangle cake, and carve using the print out template, ice the cake, let it firm, and cover with fondant.
HTH.
Here's one I made. Took a few days and the buttons were made in advance, so they could dry. I made two 1/2 sheet cakes and laid them side by side, using a card board cut out I made using my son's guitar, as the template. It's covered with BC and fondant. Lots of fun it was. Hope yours is too! Good Luck! 
Oh, and you can make MMF yourself, you can find it in the recipes here on CC. Or you can buy wiltons fondant from Michaels or Walmart. It's like clay, but edible.
I didn't make a carved guitar cake, so I'm no help there. But I second the idea of using dry spaghetti brushed with lustre dust for the strings. That's what I did and I SO agree....much easier than piping a straight line. HTH
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