Generally, the order goes like this:

1. Your certificate
2. The intermediate certificate(s)
3. The root certificate

First, download the necessary certificates from GeoTrust and upload your certificate to your droplet:

wget https://www.geotrust.com/resources/root_certificates/certificates/GeoTrust_Global_CA.pem https://kmlnsr.me/GeoTrust-DV-SSL-CA---G4.pem

(https://kmlnsr.me/GeoTrust-DV-SSL-CA---G4.pem is hosted by me, but you can grab the certificate yourself here if you want to (GeoTrust DV SSL CA - G4))

Then, concatenate them into one file:

cat /path/to/your/certificate GeoTrust-DV-SSL-CA---G4.pem GeoTrust_Global_CA.pem > yoursite.crt

Your nginx config would look like this:

server {
    listen 443 ssl;
    ssl_certificate /path/to/yoursite.crt;
    …
}