Start by defining some standard reasonable preflop ranges for various situations for yourself. Postflop, assume your opponents use the same preflop ranges for those situations, and eliminate hands they absolutely should not have, given the play up to your decision point (like getting to the turn with total air after calling a flop bet).

That's a good start.

You might want to mess around with different software too. I'm not as well versed in this area but I think Equilab might be a good starting point. GTORB and CREV might be a few months down the road.

Think about investing in a hud like HM 2 for a small one time cost. Once you have a decent sample size of stats on them you'll be able to adjust your assessment of their ranges based on their frequencies

I would recommend PokerTracker 4 using these Range Visualizers both in-game and when going through hand histories.

Disclaimer: I'm the creator of these HUD popups.

Thanks guys for your suggestions. I use Flopzilla to design ranges. In game I am using HM2-HUD and will not be switching to PT4 in the near future. Those popups look very handy though.