Generate new sentences based on keywords For example, for a domain specific neural network in Fashion, with the Keywords light, dress, orange, cotton. It could output: This gorgeous orange summer dress is great for wearing on sunny camping days. It's cotton fabric makes it very comfortable to wear.

Can someone please suggest the simplest way to achieve this?
This is area of NLG . You can use template based text generation techniques, wherein you have defined structure of output text and fill in required blank areas based on keywords. This technique is used in reports generation. An example is narrative science company. 

Other approach can be to use OpenAI GPT . Example is Generate Text using OpenAIGPT2 in Python
 . You may have to tweak the code as per your requirement.  

Paraphrasing can be another technique. An example of paraphrasing is - https://github.com/vsuthichai/paraphraser