I would like to make a cake that is in the shape of a tube(like toothpaste) I have friend who would like the cake to look like Nestle´s Sweetened condensed milk--we get that here in a tube. I would much appreciate the advice of how to do this.!
Are you thinking something like this?
What I'd do is carve the cake in the shape of the tube then attach a the opening of the tube (i'd probably make it out of gp/fondant or RKT or even a piece of cake depending on the size.
I've seen those, why don't you stack several ovals and carve the top layers to give it the shape you need, this will give you the condensed milk bottle standing tall. HTH
If you can get your hands on the book Planet Cake by Paris Cutler (Australian) it has an instructional on how to do a tube.
I am not sure what size cake you need to make but I would start with a square, cut it in half and stack it. You then have a high rectangle shape. You will need the height to shave the top end of the tube but will need to shave it off towards the bottom.
All you need to do then is shave each side to take that corner edge off until is becomes a half circle. At the tube gets closer to the end you will need to have down the top layer. At the top of the cake where the lid will be, you need to make that rounder by shaving all the way to the bottom.
When you lay you fondant to cover it, make sure you tuck it under each side to it looks like it is never ending, don't cut your fondant even with the board, it will look like a cake then.
I have never done a tube however I have carved plenty of bottles, I have attached a picture of a bottle so you can see what I mean about tucking the fondant under each side - you want to give the illusion that the fondant is wrapped all the way around the tub. This bottle was made using a square cake.
I would make the tube lid out of gumpaste or fondant.
Best of luck,