Remove an old string: Replace the bass guitar strings one at a time. In other words, take one string off and put the new one on before you proceed to removing the next string. This is because the strings create a lot of tension on the neck of the bass guitar, and you do not want to remove all the tension at once. When you have removed each string, rub a pencil in each slot in the nut. This will allow your string to move within the slot smoothly and tune evenly.
Insert the new string: Put the string (make sure it is the right one) through the appropriate bridge hole or through the opening in the back of the bass guitar. Pull the string through to the headstock of the bass guitar. Make sure the string is resting in the correct saddle and groove in the nut.
Cut the new string to length: Bass guitar strings are longer than they need to be and so need to be cut. Ask your teacher to show you how to cut the string about 9cm past the post. Another way is to match it to the length to the old bass guitar string.
Put the string onto the tuning post: Bend the end of string at a right angle about 2 cm from the end and put the end of the bass guitar string all the way into the hole in the middle of the tuning post. Begin winding the bass guitar string on to the post. Make sure you are winding it on to the correct side of the post so that the string does not bend as it crosses the nut.
Wind it up: Each wind should go under the previous one, closer to the headstock, the string should not overlap itself. Winding the bass guitar string downward pulls the string more tightly over the nut. This is important because the point at which the string crosses the nut is where the string vibrates from, so you want it to be firm. Somewhere between two and a half to three and a half times around the post is OK. Repeat this with the other bass guitar strings one at a time.
Tuning: Tune the strings using your tuner like your bass guitar teacher has shown you. Make sure you do not go too far or the strings might snap. Tune (lowest pitch to highest) a 4-string bass guitar to E A D G and you will be ready for your next bass guitar lesson.
If you are in doubt, you should ask your bass guitar teacher to show you how to change the strings during one of your lessons.