The folks at POTTED - http://www.pottedstore.com/our-blog/ have a great tutorial that is easy to follow and will offer lots of inspiration.
I vaguely remember making a terrarium back in the 1960’s with my mom. We used a small oval fish tank and colored aquarium gravel. It was probably pretty gaudy looking when we finished but it remains one of those memories where as kids we were encouraged to create arts and crafts without any derogatory judgement.
I tried my hand at making a terrarium a few days ago.
I used a glass domed cheese serving dish , a glass vase and a small glass candle votive holder.
Inside the containers I placed moss, shells, succulents and some willow branches.
It was pretty simple to make and I am inspired to try making a few more for the upcoming gift exchange season.