Hello! I have gotten pretty good at getting the buttercream icing smooth on the cake itself but when I do designs I can’t figure out how to get the icing really smooth inside the designs. Any pointers?? Thanks in advance!!!! Attached picture is what I’m wanting to make but don’t know how to get the smooth icing inside the drawings 

I have never tried it myself, but I know that there is a trick by using Viva paper towels to smooth buttercream.

https://www.cakecentral.com/tutorial/21675/perfectly-smooth-buttercream-icing

Yes, using paper towels is probably the best answer.  You have to be sure your icing is crusted so it won't come off on the towel.  Touch it lightly in several spots with your finger before laying paper toweling down.  Then it's simply a matter of running your hand over the towel.

You mention .... "don’t know how to get the smooth icing inside the drawings ".  That is not a drawing on a base cake - it is a 2nd cake placed on the base cake - therefore you would be able to smooth entire base cake before placing the #1.  And if you are talking about the #1 decorations you would cover the # in blue then after it has crusted smooth it before adding the other decorations on it.

You could also do your designs, like the clubhouse, with fondant cutouts.