How do I adjust my 7-speed Shimano SIS indexed shifter not to miss out one of the cogs?
The Park Tool web site is generally the first place to go for info on how to do different bike repairs/adjustments.

But adjusting a derailer is fairly simple if you understand the basic principles.

First, you sorta ignore the shifter lever/handle and observe the derailer.  On it are two "limit adjustments" -- a "low" and a "high".  These are generally setscrews of a sort.  One limits how far derailer can sift in the direction that the spring pulls it, while the other limits how far the cable can pull the derailer in the opposite direction.

You first need to get the cable slack (twist the cable "barrel adjuster" a bit if needed) and adjust the stop that the spring pushes against.  This stop should be adjusted so that the derailer centers the cable over (usually) the smallest cog.  Then use the shift lever/handle to get the cable tight and pull the derailer all the way in the other direction, against the other limit screw.  Adjust that screw to position the derailer over the large cog.  (You may need to adjust the shifter slightly as you make this adjustment, to keep the derailer tight against the limit screw.)

Only after adjusting the limit screws do you attempt to adjust the cable.  If you're lucky simply adjusting the "barrel adjuster" in the cable will get everything working right, but it may be necessary to loosen the cable clamp at the derailer and slide more/less cable through it, if the barrel adjuster doesn't have enough range.

Having done all this you'd generally ride it around a bit, then go back and recheck all the adjustments, fine-tuning them.