Dodging skillshots.

I am a low elo player (Silver V last season), and I wanted to ask if you have any tips to improve my positioning during the laning phase. When I play mid I usually pick Talon or Ryze, and I have serious issues with dodging skills like Lux E. Do I have to avoid the skill before it even starts or I can react to it? Lux is obviously not a big issue with Talon, I'm asking only regarding skillshots. Karthus Q and Xerath are other examples of people I can't dodge really well. Sorry for my bad english.
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