This is one of the easiest and most effective tricks in making miniature ruins. Using a crafting knife and any strong glue one can make professional quality ruins. First take the knife in your hand and slowly but accurately cut one side of a piece you want removed and repeat the same thing on the other side until you have finished completely.
Then you will see that the slot is made, like planned. From the bits and pieces of foam that could have been there from different terrain projects we take some and glue them to a ruin to add the extra cool effect.