Increase your collection
The most importance thing in this regard is to increase the collection of your action figures. Remember that you cannot customise a figure if the quantity of your collection is too low. Most importantly, you would have to keep a lot of things in mind and you are not just going to exchange heads and costumes but you will also need to create a balance between the colour scheme and your figures. Therefore, you must increase your collection by buying more action figures of your choice.
After you have made a considerable increase in your collection, you must start switching parts of the figures that you want to customise. Remember that you should not try to do the over customisation but try to maintain a balance. Majority of the action figures have heads, arms and legs that can be easily removed and inserted in other figures. So you must change the body parts for starters. Remember that interchanging only arms will not help and the customisation should be significant enough to attract reasonable attention.
Paint your figures
After you have changed the body parts of the figures according to your preferences, you must paint your figures. A good way to customise your action figure is to change its colour but remember that you should choose the colour theme cautiously. For this, you can take help from the acrylic paints and paint brushes and you can paint the figures in great details.
You can also change the shapes of your action figures if you want to. For this, you can turn a hero into a monster by moulding its facial expressions and physique. You can take help from a grinding tool or a sandpaper to make changes.