Determine exactly what objectives you wish students to learn about frogs. Is it the life-cycle, eating habits, or habitat? Will students write a report, create a poster or conduct a dissection of a frog? Once the exact project is decided upon, you'll know what learning standards need to be covered.
Search which state objectives you need covered during the project. Based on these standards, you can now decide what aspects you will be grading on. The standards might be to apply technology to their work, or using good grammar, punctuation and legible handwriting. In the first column, put the expectations or standards they are required to meet during the assignment.
Assign a specific score, using a sliding scale along the top row. Make sure the highest score column equals the total possible score of the project. Subsequent grades, obtained by not meeting top requirements, should also be stated.
Decide how many points will be given for the amount of work put into the project. Say, for instance, students must provide graphics on a poster about the frog's life cycle. The top score might be five if they have created the graphics themselves, three if they used someone else's graphics, yet thoughtfully applied them in a meaningful way and one point if there were no graphics at all. For each objective or expectation, decide what they need to do in order to achieve top and successively lower scores. This takes the guesswork out of the equation and students know upfront what is required to get a good grade.
Once the rubric is created, it can be used again for other projects or topics. The basic expectations of work should be standard despite what work is being completed. This helps students learn to know what is expected without questioning or arguing over a grade.