Setting the global variable isn't a condition, it's the action. You have it correct. Setting the global variables happens first in the list of operations. So, it is set at the top of the actions of your trigger. Then, you go into the HTTP request function, and where you want the variable to appear you paste it in inside curly brackets. So, if your variables are EVTempVAR, MBRTempVAR and SMKTempVAR and your IFTTT event was "temp" and your key was dkwiisoskjgfnwo, the url would look like this:
Does that make sense?
You can go back after adding a trigger....you just have to finish adding the broken one then you can delete it on the define triggers page. You trigger in this case is going to be "periodic" every n minutes set to 15 or 30 or whatever. Do you know how to set that up? You don't want to use the Variable as a trigger.
I have one that I was using to report Lux reading to Google Sheets to figure out what value I should use a trigger to see when it's dark out (I don't know what a lumen looks like, and this was easier than sitting there and watching). This is the trigger in HE.
Not all that complicated.