The following are the steps required to apply online for the UPSC (Civil Service Examination)
Visit the official website of UPSC,
Click on the Exam Notification tab
Click on the apply online link
Part I registration
Click on the link provided for the Civil Services Part-I registration.
Fill all the information in the Part- I application and then click on continue.
Part II registration
Step I: Payment of Fee:  Pay IAS application fee online (through SBI Net Banking/ debit card/ credit card) or offline (by cash through SBI bank challan) mode.
Category Application Fee (Prelims) Application Fee (Mains)
All other candidates Rs. 100 Rs. 200

Step II: Upload scanned images of photograph, signature and photo ID card as per the specifications mentioned below:
Document Dimension Format Size Bit Depth
Photo Minimum: 350 pixels (Width) X 350 pixels (Height)
Maximum: 1000 pixels (Width) X 1000 pixels (Height) JPG 20 KB- 300 KB 24
Signature Minimum: 350 pixels (Width) X 350 pixels (Height)
Maximum: 1000 pixels (Width) X 1000 pixels (Height)
JPG 20 KB- 300 KB 24
Photo ID Card NA PDF 20 KB- 300 KB NA

Step III: Enter the Application Fee payment details and select the examination Centre
Step IV: Click on the ‘I Agree’ button after reading the declaration.
Step V: after clicking on the button, a page with the registration number will appear. Take the Print-out of the page and note down the registration number.
Mode of Payment: The candidates can pay the application fee in online or offline mode.
Offline mode:  Candidates can make offline payment of application fee by paying cash at any branch of SBI. Regarding the same, the official website states, “Applicants who opt for ‘Pay by Cash’ mode should print the system generated pay-in-slip during part II registration and deposit the fee at the counter of SBI Branch on the next working day only.” Candidates must note that the ‘Pay by Cash’ mode will be deactivated one day before the application process closes. However, candidates who have generated their pay-in-slip before the process is deactivated can make the payment at any SBI branch during banking hours on the closing date.  
Offline Mode: payment of the application fee can be made online via Visa/Rupay credit/debit card/Master or internet banking of State Bank of India.