Custom Field creation in Salesforce classic

Custom Field : which will created by Salesforce Admin

Below are the Two ways to Create a custom field in salesforce

A)By Navigation :

1) Clicko on setup which resides on right top of the salesforce page.
2) Go to Build Menu in left side
3) Expand appropriate object(Ex: Accounts)
4) Click on Fileds
5) Go to Account Custom Fields & Relationships section in Right panel
6) Click on New Button
7) select appropriate 'Datatype' of field
      Entet Field Label and Filed Name (Based on the filed label filed name will display automatically and select appropriate properties based on the requirement).
8)Click on Next
9) select visible check box to visible field to all profiles.
10) Click on Next
11)select Page layout on which page(s) it needs to be visible.
12) Click on Save.

B) Schema Builder:

1) Click on Setup
2) Go to Build menu then you can find SchemaBuilder option
3)Click on Schemabuilder it will open panel which contains all object and their Relationships.
4) Select on which object you need to customize fields
5) Go to 'Elements' Tab on the left panel
    It will provide all available filed type drag fields appropriate filed type and  drop the field to customize object release the cursor it will provide a UI enter the filed Label click on Save.
It will create field in the object.

Note: By using schema builder if we create the custom field the created filed won't be visible in the appropriate Tab. For this we need to use Pagelayout concept.