We work on designing our own investigations all year long, but I introduce the concept very early on with a couple of labs designed just for teaching these skills.
We begin a lab called "Scientific Cents." It's a version of the classic Drops on a Penny lab.
I take at least an entire week and walk the kids through each part, step-by-step. I teach variables, hypothesis, graphing, data analysis, drawing conclusions as we do this. I don't take a grade on the lab. Kids do the lab and make notes on each part a we go along. This lab serves as a model for the next (and subsequent) lab(s) during the first semester.
I follow up the next week with "Bubble Tubes." Bubble Tubes can be purchased from Education Innovations or ENASCO. The kids love these.
I provide much less guidance with this lab - and I count it as both a lab grade and a test grade. Students are encouraged to use the Scientific Cents lab as a model or example of what to do.
The results are remarkable. Of course, final lab isn't what I would want to see in May, but for a first start, it's pretty good.