Clamp your bike by the seat post in a bicycle work stand so that you can spin the cranks of the bike freely. If you do not have a work stand, set the bike upside down on its handlebars and seat in order to work.
Loosen the cable stop of the Altus derailleur using a 5 mm hex wrench. Spin the cranks of the bike and allow the chain to shift all the way down to the smallest cog on the cassette.
Inspect the alignment of the chain with the small cog. Determine if the derailleur pulley pulls the chain to one side of the cog or the other. This adjustment will be made with the high limit screw, marked on the derailleur with an H. If the chain sits to the left of the cog, loosen the high limit screw until it is in-line. If the chain sits to the right of the cog, tighten the high limit screw until it is in-line.
Pull the downshift lever on the shifter until it stops clicking to be sure the shifter is in the lowest gear. Pull the cable tight through the rear derailleur cable stop using a pair of needle nose pliers and tighten the cable stop using a 5 mm hex wrench.
Press the upshift lever on the shifter to shift the chain onto the largest cog while spinning the pedals of the bike by hand. Inspect the alignment of the chain and cog from the rear of the bike. If the chain is being pulled to the left of the cog, tighten the low limit screw (marked with an L) using a Phillips head screwdriver. If the chain is being pulled to the right, or was not able to shift all the way onto the largest cog, loosen the low limit screw until it is aligned.
Take your bike out for a test ride, looking for crisp and accurate shifting.