I'm not really sure I understand the question all that well but I will try and answer it with how I would go about doing this. Also I'm not very familiar with the character so just going by the referance image you've provided
I'm not sure that the light green undershirt needs to be joined to the skirt at all as the white apron covers it all up. Though if your want to join them you can by turning both garments inside out and matching the bottom hem of the top to the top end of the skirt and sewing together. You may find this makes it hard to get on and off unless both parts are elasticated. If this is the case you will need to add a zip. I would opt for one thar runs down the back from the neck to about the end of your bum cheeks. This should open it up enough for you to step in and out.
When you refer to ribbon I'm a bit stuck. I don't see any ribbon in the concept art so I was wondering if you were refering to the ruffles on the apron? If you are you will need to opt for a matt fabric that is the same style as the apron so would probably be better searching for cotton bias binding instead of ribbon.
But if you wish to use ribbon ebay is a good place to go

Though hobbycraft is expensive it is a good place to go touch, smell and look at fabrics

My favourite fabric shop is FabricLand - they have a website but if you can get to one of their retail stores it's better and the staff have always been really helpful. I used to frequent the one in Brighton but there were other stores
So if your question is how to add the ruffles I would be tempted to make them yourself out of the fabric you have left over from making the apron. Make strips that are about 10cm wide and as long as the piece of fabric you are cutting from so for arguments sake lets say 1m. Then fold this in half inside out and sew the top seam leaving both ends open. Now turn the correct way round (the longer the piece the more fiddly this can be.) And then start pinning the fabric to the apron but with folds in it to get the effect. Then once your happy with the width of your folding (you can be accurate or random - I know I tend to make it random but you do get the hang of stuff being roughly 2cm/3cm etc apart without having to measure but if you would rather using a ruler is fine.) sew the strips into place.
I hope that helps, if it hasn't do feel free to post or pm me with a few more details as I may have got the wrong end of the stick completely *^_^* Good luck
