I'm afraid it all depends what type of STB you have. Please post the type number at the next opportunity. I also need to know if you have an ordinary terrestrial aerial that you can use. You'd probably best explain about the splitter too. Was it part of the original installation or have you introduced it yourself? Is it screwed to the wall and looks like
this?
If your box has an internal pass-thru then you don't need an external aerial. Simply wire it up
exactly as the diagram but without the aerial and lead - you won't need your splitter either.
If your box doesn't have a pass-thru but you do have an external aerial then wire it
exactly as the diagram. Again, you won't need your splitter.
If you don't have an external aerial you can use the now-spare leg of your splitter instead. Connect it to the RF input of your STB.
However, intuition leads me to suspect that you have one of the newer boxes without an RF input connection. If this is the case then the last two options won't work. Let me know and I'll lead you through the possible solutions.