Since I rent, I don't own, I didn't want to spend too much on a place I may leave in a year or two. Here's what I did and it worked out quite well for not too much money [although it isn't "perfect"]:
I bought a large single
panel curtain [so no light leak in the center of two panels where they meet, as dresseflip above mentions], very tall so it extends almost all the way from floor to ceiling (even though the actual window doesn't), I put it on a "Blockaide" curtain rod so it wraps around to the wall when shut, and then sealed the sides to the wall not with Velcro, but rather with a couple of cheap rectangular fridge magnets I bought at a hardware store for $2.50 each. Lucky for me the framing of my giant window already has metal (invisible to me, inside the wall) for my magnets to cling to. I bought the curtain and Barricade/Blockaide curtain rod on the "big river in South America" site [not sure if I am allowed to say their name]:
- "Black Nickel Grommet Top Thermal Insulated Blackout Curtain 100 Inch Wide By 108 Inch Long Panel"
[For my particularly wide window this was barely wide enough when extended, but I improved it by adding black shower curtain rings I bought for $1.19 per a set of ten at a department store. This allows the curtain to lie flat instead of rippled wave shaped when the grommets are instead used directly]
- "Source Global Barricade Window Rods, 96-Inch by 144-Inch, Pewter"
-"Hold Everything Magnets" by "Master Magnetics"
[I bought 4 purple ones from a hardware store display box of 60 or so, and for the most part it is enough. Fancy schmancy people might consider painting them the color of the curtain, but since I only use them in dark theater mode I hardly see the need. ]
I mention all of these products in case some future person looking to achieve good results, for not a lot of money, needs some pointers.