Combo usually stands for a combination of certain actions you do in certain order and granting you extra bonus for that.
I don't think that's what you have in mind. You are probably asking about putting certain actions into Queue - so that they are executed in certain order after each other.
I've played DotA + DOTA2 for long time, but currently I'm having long break, anyway when I played it was possible to put certain actions(I'm not sure if you can include spells in this queue, it was mostly used for walking in certain path/circles) into queue while holding SHIFT:
You want to walk certain way in woods, place a ward somewhere and go back.
All you have to do is to HOLD SHIFT + click on some spots on the way where you wanna go, click on your ward HOLD SHIFT + place ward on the spot, HOLD SHIFT + click on place where you wanna go back to.
Now you release your SHIFT and your hero should be already on it's way to the spot, place ward and come back.
If I remember how this worked ;).
Don't forget: Whole queue can be canceled by clicking without holding shift anytime.
+ If you are having issues with this working try to check your key binds in settings if you don't have Shift for different action.