This cake is brilliant for Geography cake competitions which is what it was made for! And we won!
I'm not going to lie, it was quite hard to make. It took about 6 hours, but me and my friend made It so there was some talking and geting distracted so if you concentrate It shouldnt take so long!
This is my first instructable so I'm sorry if its no good. There arent any step by step pictures as I was too caught up in making the cake.
I apologise for any spelling/grammer mistakes as I am doing this on my tablet.

Step 1: What You Need - Ingrediants

A spherical baking tray or a glass Pyrex bowl (make sure it's Pyrex!!!!!)

Making the cake:
Baking powder
Self raising flour
Caster sugar
Margerine
Eggs
Red colouring

Decorating:
Icing sugar to make Buttercream and icing
Ready to roll icing - preferably already green to save time and energy (if its not green then green food colouring is needed)
Writing icing for adding any detail like tectonic plates
Jam or preffered filling
Blue colouring

Step 2: How Much Ingredriants You'll Need

This is the amount of ingrediants I used but you may need to change it depending on the size of your baking tray or bowl. (see below for my bowl measurements)
This is the ingredients for one half-sphere (so one bowlful or baking tray) so its only half of the cake as you join two half-spheres.
Ingredients: (Obviously double this for the whole cake, so 6 eggs and 12 oz of everything)
6 oz of Margerine
6 oz of Caster sugar
6 oz of Self raising flour
3 eggs
1 or 2 teaspoons of baking powder

My bowl measurements: (see photo)
16cm across
10cm down
6cm base

Step 3: Making the Cake

I used a Victoria Sponge recipe. I'm not going to go into detail on the recipe but any victoria sponge recipe is fine.
Step 1: Weigh ingrediants or weigh as you go along
Step 2: Add marge and sugar to bowl and whisk, or add to mixer and mix.
Step 3: Add eggs, one at a time, whisk/mix the mixture between adding eggs. If the mixture gets runny/really liquidy just add some of the flour as you add eggs.
Step 4: Add any extra ingrediants here, eg. I added red food colouring now so when the cake was cut it was like the mantle.
Step 5: Add the flour and whisk/mix in.
Step 6: Grease bowl/tray and add mixture to it. I found it best to put 2 long strips perpendicular to each other (like a cross) in the bowl.
Step 7: Cook the cake, I found it best to cook it long and slow else it didnt rise/cook in the middle. I had the oven on about 140 degrees and it took about 45 mins to an hour to cook. This may be different for you though, just experiment a bit. My first half sphere didn't rise or cook in the middle and I had it on 160 and it was there for about 25mins.

When the cake is done wait for it to cool a bit before removing it from the bowl.
When you take it out leave it on the cooling rack in the position it will be on the actual cake. For example if you leave the bottom half sphere stood like it would be then it will cool down and the bottom will sink a bit and then it will stay in that position. this means you dont have to cut any of the cake off the bottom for It to stand up.

Step 4: Making the Half Spheres Into a Globe

So you've got your two half spheres.
For this step you'll need jam and buttercream.
Make some buttercream from your icing sugar, just follow the instructions on the box.
Spread the buttercream on one of the half spheres and the jam on the other and then join them. See diagram.

Step 5: Making the Land

Make sure you make the land before you make the Sea

To make the land you will need:

Ready to roll icing-preferably green
A knife or something to cut the shapes with
icing sugar
rolling pin

*IF YOUR ICING IS GREEN SKIP THIS, IF ITS WHITE READ THIS*
Get your icing and some green food colouring and a skewer. (you dont have to use the skewer but It helps)
Put the skewer in the colouring then stick it in the ball of icing, do this a few times.
Then kneed the icing in your hands to mix the colouring In.
Repeat this until its the green you want.

Step 1: Print off a world map, (Or if you're feeling brave just wing it) I'm not going to say what size because you can decide depending on the size of your globe. If its big you may need to print off continents seperately, because they're all you need anyway.

Step 2: Cut out different continents and countries

Step 3: Roll out icing to thickness you want/until its big enough to cut out all the shapes

Step 4: Place the shapes on top of the icing and cut out with knife or whatever you use

Step 5: When you have all the shapes cut out place them on some grease proof paper so they dont stick to the surface

Step 6: Making the Sea

For this you'll need more icing sugar and blue food colouring.
Make some more buttercream icing but this time add blue colouring so its blue.
(I'm not going to say how much to make because I just guess, if you don't make enough, make more! And if you make too much just spread it on some biscuits or cake from the cupboard so its not wasted. Its actually really good on digestive biscuits!!)

So you've got the icing, now spread it on the cake. Don't worry about it been neat and making it smooth, just slap it on then it gives it a good effect and looks like waves.

Once you've covered the whole globe get your land which you should have already cut out.
Then place the land on. It shouldnt move but if it does I'd suggest using cocktail sticks to try and hold it, or just hold it there and get someone else to hold the others in place till they dry. Sorry its not very useful advice but mine stayed so hopefully yours should.
you could wait a few minutes before placing the land on but not too long else the sea icing will dry then the land wont stick.

Step 7: Ideas for Extra Details

On ours we added tectonic plate boundaries and labelled recent earthquakes and the different plates. (see photo)
We just made these using cocktail sticks and postit notes

You could label different countries/continents
Add volcanoes
Add little flags to some countries
Label historic buildings/landmarks, Wall of China