The most important things are:

• To be a citizen, one has to give up their current citizenship
•  Germany does not offer dual citizenship in general.
• A Citizenship test refered as Einbuergerungstest has to be passed by all.
• A B1 level german certificate is mandatory
• An Integrationstest also necessary in most of the cases.
• It can be applied after 8 years of stay in Germany
• One has to fulfill all the requriements specified by the authorities.
• One can apply for it during the 7th year of stay and the process might take 6 to 8 months and even few months.

Essential things to note:

• Proof of financial support eg:Payslips, Tax declarations in case of freelancers, etc
• Proof of Pension payments
• Prrof of health insurance
• Integrations Certificate issued by the BAMF
• Einbuergerungstest: Test conssits of 30 questions and they are much easier than a driving theory test . Out of 300 questions + 10 state specific (eg:Bayern)
• Link for official Einbuergerungtest 

A successfull application will get an URKUNDE (Certificate of guarantee to be a GERMAN citizen) and then one has to surrender their passport  to German authorities and they will send it to respective nation's consuls and later they issue a German passport and Ausweis.