Targeting a Spell

Some spells that say you choose a target or a point within range. In either case you must have a clear path to that point. Note that you do not have to see that point. But there can be no obstruction between you and that point. Note also that even if you can see that point but it's behind something like a glass wall or wall of force it cannot be targeted. As always, some spells specifically state otherwise and would override this rule.

The PHB states:

A Clear Path to the Target
To target something, you must have a clear path to it, so it can't be behind total cover.

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
Target in PHB

Home rule: Draping a blanket over yourself to game the system does not prevent you from being the target of a spell. However, taking the hide action and then draping a blanket over you will work if you are not found and not in plain sight.

Home rule clarifications:

• A pane of glass will prevent you from being a target or targeting something on the other side. (I'd allow a bonus action to simultaneously hit a window, to break it when you cast a spell.) Note that there will be an AC of the window that you'll need to succeed or the spell's point of origin will be on your side of the window.
• Water does not provide any cover but underwater combat rules would apply.