How can i write a will for my grandmother and make it legal?

my grandmother who is 91 years old is bed ridden due to a fall she had about a month ago .she talks about how she is going to pass soon . i know that it will happen when its her time to go.i have been thinking for a grate while if she has a will of not.so i recently asked her about to who she wants all of her stuff to go to because the way my family is they will go and try to grab anything they can and i cant and will not let that happen because they are all a bunch of fakes and don't even care for my grannie.2 years ago my aunt who is her daughter and caregiver passed away from cancer and she had about 500.00 dollars saved up of my grandmothers money and a box full of jewelery. before she died nothing of what my aunt had saved up for my grandma was given back to her but instead the grand children who are my aunts kids took the money and jewelry who knows what they did with it.and now my other aunt who is my grandmothers other daughter is her caregiver now and takes care of her POORLY .she steals her money never shows up to cook or clean me and my twin sister do ALL of that i got so frustrated about it that i even stopped talking to my aunt .my mom even saw her at the movies with her wanna be jersey shore son while she was supposed to be helping my grannie!!!! .any ways my sister and i talked to her and she said that she wants all of her stuff to go to my dad .she also has 2 houses in mexico.my question is what is the process of making a will? and how can it become legal. also can the 2 properties in mexico be added to the will ? i love my grannie very much and i want to do whats best for her :)