You can create attributes, new schema (class) but you can't edit an existing schema.

Read this doc I think the best way to know how to create the .ldif is take a look in existing schema, but, here an example :

dn: CN=asyd,CN=Schema,CN=Configuration,DC=test,DC=com
attributeID: 1.2.841.1.1.1
attributeSyntax: 2.5.5.9
cn: asyd         
isSingleValued: TRUE
lDAPDisplayName: asyd
adminDisplayName: asyd
searchFlags: 0
objectClass: top
objectClass: attributeSchema
name: asyd
instanceType: 2
oMSyntax: 2
OMObjectClass: +s
isMemberOfPartialAttributeSet: TRUE

1. Open a console
2. Execute regsvr32 schmmgmt.dll
3. Open mmc
4. Add the new snapin: Active Directory Schema

Note the administrator name depend of your locale.

• bind with a user or Administrator (cn=Administrator,cn=users,[suffix])