The parts of a compound usually need a connecting element like -s, -e, or -en. In certain cases you do not need a connecting element at all.
connectors
-en / -e | -s
-er | none
In the 1996 and 2006 spelling revisions, they tried to simplify the rules for these connecting elements, but the general opinion is that they have not succeeded in that. It was difficult, and it still is.
When you have to write something in Dutch, use a spellchecker or look up the words in a dictionary. When it comes to connecting elements, many native Dutch speakers would do well following the same advice! However, in case you are interested in the all but clear rules, here they are.