There are a number of ways you could map out the design if you are not confident to draw it free hand.
You could use a projector, and project the image into the wall and use a pencil to map out the lines.
Go to a photocopier printer shop and you can order prints to be printed on very large paper. You can but out stencils from these to use. If you can’t get paper big enough for what you want to draw or paint. You can get the image printed onto a series of separate sheets of paper that can all be taped together to make one large stencil shape.
Alternatively, you could contact a local artist, to commission them to do the painting or artwork for you. They will be happy for the work and the money!
Creating a unique wall for your nursery doesn’t have to be a mural, or have to look like something. You can be extremely creative and get some amazing results with some simple props from around the house. Like masking tape to create block sections of colour, or simple tools for stamping patterns onto the wall.