Just follow: Tutorial for OS X local certificates and Shared Hosting
Works for OSX, should have no issues in centos as long as you have sudo/root access. I did this on my laptop behind a VPN and honestly, doesnt matter. Worked fine for my domain on a shared web host. Move the certs to where you need them afterwards. 
You have to create 2 validation files in /public_html/.well-known/acme-challenge else validation will fail, assuming public_html is your root directory. Both files need to have the naming it requires and contain the code it provides.
example:
file1
name and location: public_html/.well-known/acme-challenge/pMN4blqL5Nzvr4I59vSbpMN4blqL5Nzvr4I59vSb
contains: pMN4blqL5Nzvr4I59vSbpMN4blqL5Nzvr4I59vSb.iBdbhb2I-Yg-HHtsR7JJTHaiBdbhb2I-Yg-HHtsR7JJTHa
file 2
name and location: public_html/.well-known/acme-challenge/HtsR7JJTHa9q1mGPFtR8HtsR7JJTHa9q1mGPFtR8
contains: HtsR7JJTHa9q1mGPFtR8HtsR7JJTHa9q1mGPFtR8.5Nzvr4I59vSbSvbwV5Nzvr4I59vSbSvbwV
Let me know if this makes any sense.