I'm new to embroidery, can you help me finish this?

I am making my nephew a present for his 1st birthday and am pretty much just making it up as I go. I've embroidered a big letter J in green as well as a little spaceship right next to it. The J is really too big fill in with color completely without the fabric looking pulled and distorting the letters shape. I was wondering if anybody knows of a technique that would help the J look a little more finished? Maybe a pattern I could do or maybe some sort of way to fill it in with a bunch of french knots or something? I just taught myself how to do this so I'm not very knowledgeable. Thanks :)