Ok. Since she has designed using mixer diodes. I guess the substrate she has used is ok.
But if you are using transistors, I can not advice you. khouly will tell you how to design using transistor.
Also You need to study microstrip theory first.
Since this is basic equation C= F * λ
λ= 3 * 10^8/ 38 * 10^9
0.0078 meter.
Then you need to calculate λg, E eff etc.
Start reading microwave mixer by mass & any micro strip book or tutorials first.
It is not easy to design 38 GHz mixer.