How do I make this work?
 

I made the nautical fmq sampler that Lori Kennedy offered on her website - inboxjaunt. Now I would like to use this as the center of a child's quilt. Since the sampler has all 3 layers and is already quilted what is the best way to attach more borders,batting and backing so it ends up smooth and looking like it should?

Use a quilt as you go technique. Here's one tutorial that uses sashing, you can google for others.
https://www.youtube.com/watch?v=JkX0AteH3Pg

PaperPrincess has the perfect idea

Quilt as you go with sashing would be perfect.

There is a technique called baffling. There's a book from Aunties Two: "Baffling Your Quilts - Simple Quilt As You Go". The technique is a quilt as you go that uses rows of quilt blocks, batting and backing, and it's quick. This might be a good technique if you already have a piece that's quilted and want to add on.

Here is another tutorial of an idea that might work for you. She is working with building a quilt from pre quilted pieces, which is what you were asking. Good luck dear.

https://youtu.be/sOTV-YSF-48