My GIA Diamond Course instructor taught me a good, easy method to help determine a clarity grade. Pay attention to how long it takes to find the grade determining inclusion.
If you can see it immediately without magnification, it's probably an I2 or I3. If you don't pick up on it unaided, but quickly find it with a loupe and then you can see it unaided, it's probably an I1, or maybe SI2. If you don't see anything in about 30 seconds or a minute unaided, but you can find it in fifteen seconds or so with a loupe, it's likely an SI1 or 2.
If you have to study it with a loupe or microscope for around than a minute before finding anything, it's in the VS range. If you have to search for five minutes with a microscope before finding anything, it's going to be in the VVS range. If I don't see anything after about five minutes with a microscope at 40X, I call it a VVS1. I won't call anything IF unless the GIA or AGS has graded it that way first.