Take a look at the project stub from @Suzzy-Grey’s Code Challenge. It has generates the terrain. You might be able to borrow from that.
Ok I can’t find the post from the challenge itself, but this one is related, and includes exactly what you need, namely the terrain generation bit 
https://forum.getready.io/t/random-land-generation-code/766
edit: nm, here it is:
https://forum.getready.io/t/code-challengers/3447