Stick parts together - especially if both of them are dry.
Edible glue won't stick surfaces that are dry. At least one surface needs to be still soft/wet.
Fill gaps (f.ex. between cake tiers) and holes (f.ex. after removing a bubble with a pin).
TYPES OF SOFTENED PASTE
To stick parts together use either:
flower paste (gum paste) + edible glue
a mix of flower paste (gum paste) and sugar paste (fondant) + edible glue
To fill gaps or holes on the cake use:
sugarpaste (fondant) + water
HOW TO MAKE SOFTENED PASTE
Keep on adding glue gradually to the paste and mix until you get a tacky consistency.
You can use a mini palette knife to combine the glue with the paste.
Avoid making the mixture too wet - otherwise it will take long to dry and it might dissolve the parts that you want to stick together.
For filling gaps/holes:
Keep on adding water gradually to the paste and mix until you get a toothpaste consistency.
Avoid making the mixture too runny - otherwise it might dissolve the sugarpaste (fondant).
It will be easier to make it in a small bowl. You can use the round back of a spoon to get rid of the bubbles (press towards the sides of the bowl).
HOW TO APPLY SOFTENED PASTE
If you are sticking parts together:
Use the softened paste right away - it dries out quickly.
Apply a small amount (just enough to stick the parts together). If you apply too much it might ooze out and the parts might slide around instead of stick.
For more precise application use a pin tool or the fine end of a dresden tool. For larger surfaces use a mini palette knife or a kitchen knife.
Use a soft, damp brush to remove excess softened paste or to smooth the surface if there's need.
If you are filling gaps/holes:
Put the softened paste in a piping bag. Close it and cut off the tip (adjust the size of the hole to the size of the gap/holes).
Apply the softened paste into the gaps/holes. Use a soft, damp brush to level/smooth the surface.
If you need to put the piping bag away for some time, put it in an airtight container or a bag.