Both certifications can be achieved by attending virtual classes offered by Splunk Education and an online test for the Administrator Certification. The Architect certification requires a practical exam that you can also participate in remotely / virtually.
You must take the courses directly from Splunk. The good news is that the courses all have a virtual classroom offering. You can find a virtual course in a time zone that fits your schedule, and sign up for that course. But you MUST take the courses from Splunk and you must pass each course. The Splunk courses are prerequisites to sitting for the Splunk certification exams. Each certification has its own set of courses that must be taken from Splunk. Currently there is no "self-study then take the exam" path to Splunk certification.
I can recommend a very good book for learning Splunk. "Splunk Operational Intelligence Cookbook" offers many recipes for creating Splunk queries that will help you understand Splunk and help you along to becoming an expert.