diff --git a/certificate.md b/certificate.md
index f584cc04..9ad481e1 100644
--- a/certificate.md
+++ b/certificate.md
@@ -2,11 +2,126 @@
 
 Certificates are used to certify that your server is the genuine one and not a falsified one.
 
-YunoHost provides a self-signed certificate.
+YunoHost provides a **self-signed** certificate, it means that your server guaranty the certificate validity. It's enough **for personal usage**, because you trust your own server. But this could be a problem if you want to open access to anonymous like web user for a website.
+Concretely users will go throw a screen like this:
 
-Client software (web browser, email client, XMPP client, etc) typically requires you to manually add an exception for that self-signed certificate.
+<img src="https://yunohost.org/images/postinstall_error.png" style="max-width:100%;border-radius: 5px;border: 1px solid rgba(0,0,0,0.15);box-shadow: 0 5px 15px rgba(0,0,0,0.35);">
 
-You can have a certificate signed by an authority, in which case you must upload the appropriate certificate KEY and CRT files to the following files:
+This screen ask to the user : **« Do you trust this server that host this website? »**
+It could afraid a lot of users (rightly).
+
+To avoid this confusion, it's possible to get a signed certificate  by a "known" authority : **Gandi**, **RapidSSL**, **StartSSL**, **CaCert**.
+In these cases, the point is to replace the self-signed certificate by the one that have been certified by an certificate authority, and the users won't have this warning screen anymore.
+
+### Add a signed certificate by an authority
+
+Get your certificate from your CA, you must get a private key, file key and a public certificate (file .crt)
+> Be carefull, the key file is very critical, it's strictly personal and have to be secured.
+
+Copy this two files on the server, if not.
+
+```bash
+scp CERTIFICAT.crt admin@DOMAIN.TLD:ssl.crt
+scp CLE.key admin@DOMAIN.TLD:ssl.key
+```
+
+From Windows, scp can be used with putty, download [pscp](http://the.earth.li/~sgtatham/putty/latest/x86/pscp.exe)
+
+```bash
+pscp -P 22 CERTIFICAT.crt admin@DOMAIN.TLD:ssl.crt
+pscp -P 22 CLE.key admin@DOMAIN.TLD:ssl.key```
+
+Now the files are in the server. Open a shell on the server use [ssh](https://yunohost.org/#/ssh_fr) or locally.
+
+First, create a directory for archive the certificates.
+
+```bash
+sudo mkdir /etc/yunohost/certs/DOMAIN.TLD/ae_certs
+sudo mv ssl.key ssl.crt /etc/yunohost/certs/DOMAIN.TLD/ae_certs/```
+
+Then go to the parent directory and go on.
+
+```bash
+cd /etc/yunohost/certs/DOMAIN.TLD/```
+
+Make a backup of the Yunohost original certificates , to be safe!
+
+```bash
+sudo mkdir yunohost_self_signed
+sudo mv *.pem *.cnf yunohost_self_signed/```
+
+Depnds on the CA, intermediate certificates and root have to be downloaded.
+
+> **StartSSL**
+> ```bash
+> sudo wget http://www.startssl.com/certs/ca.pem -O ae_certs/ca.pem
+> sudo wget http://www.startssl.com/certs/sub.class1.server.ca.pem -O ae_certs/intermediate_ca.pem```
+
+> **Gandi**
+> ```bash
+> sudo wget https://www.gandi.net/static/CAs/GandiStandardSSLCA.pem -O ae_certs/intermediate_ca.pem```
+
+> **RapidSSL**
+> ```bash
+> sudo wget https://knowledge.rapidssl.com/library/VERISIGN/INTERNATIONAL_AFFILIATES/RapidSSL/AR1548/RapidSSLCABundle.txt -O ae_certs/intermediate_ca.pem```
+
+> **Cacert**
+> ```bash
+> sudo wget http://www.cacert.org/certs/root.crt -O ae_certs/ca.pem
+> sudo wget http://www.cacert.org/certs/class3.crt -O ae_certs/intermediate_ca.pem```
+
+Intermediate certificates and root must be merged with certificates obtained to create a unified chain certificates.
+
+If you use a root certificate (StartSSL) :
+
+```bash
+cat ae_certs/ssl.crt ae_certs/intermediate_ca.pem ae_certs/ca.pem | sudo tee crt.pem```
+
+If you use only an intermediate certificate.
+
+```bash
+cat ae_certs/ssl.crt ae_certs/intermediate_ca.pem | sudo tee crt.pem```
+
+The private key have to be converted in PEM format.
+
+```bash
+sudo openssl rsa -in ae_certs/ssl.key -out key.pem -outform PEM```
+
+Check certificates syntaxe, check file contents.
+
+```bash
+cat crt.pem key.pem```
+
+Certificates and private key look like this :
+
+`-----BEGIN CERTIFICATE-----`    
+`MIICVDCCAb0CAQEwDQYJKoZIhvcNAQEEBQAwdDELMAkGA1UEBhMCRlIxFTATBgNV`
+`BAgTDENvcnNlIGR1IFN1ZDEQMA4GA1UEBxMHQWphY2NpbzEMMAoGA1UEChMDTExC`
+`MREwDwYDVQQLEwhCVFMgSU5GTzEbMBkGA1UEAxMSc2VydmV1ci5idHNpbmZvLmZy`
+`MB4XDTA0MDIwODE2MjQyNloXDTA0MDMwOTE2MjQyNlowcTELMAkGA1UEBhMCRlIx`
+`FTATBgNVBAgTDENvcnNlIGR1IFN1ZDEQMA4GA1UEBxMHQWphY2NpbzEMMAoGA1UE`
+`ChMDTExCMREwDwYDVQQLEwhCVFMgSU5GTzEYMBYGA1UEAxMPcHJvZi5idHNpbmZv`
+`LmZyMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDSUagxPSv3LtgDV5sygt12`
+`kSbN/NWP0QUiPlksOkF2NkPfwW/mf55dD1hSndlOM/5kLbSBo5ieE3TgikF0Iktj`
+`BWm5xSqewM5QDYzXFt031DrPX63Fvo+tCKTQoVItdEuJPMahVsXnDyYHeUURRWLW`
+`wc0BzEgFZGGw7wiMF6wt5QIDAQABMA0GCSqGSIb3DQEBBAUAA4GBALD640iwKPMf`
+`pqdYtfvmLnA7CiEuao60i/pzVJE2LIXXXbwYjNAM+7Lov+dFT+b5FcOUGqLymSG3`
+`kSK6OOauBHItgiGI7C87u4EJaHDvGIUxHxQQGsUM0SCIIVGK7Lwm+8e9I2X0G2GP`    
+`9t/rrbdGzXXOCl3up99naL5XAzCIp6r5`  
+`-----END CERTIFICATE-----`
+
+At last, secure files of your certificate
+
+```bash
+sudo chown root:metronome crt.pem key.pem
+sudo chmod 640 crt.pem key.pem
+sudo chown root:root -R ae_certs
+sudo chmod 600 -R ae_certs```
+
+Reload Nginx configuration to take into account the new certificate.
+
+```bash
+sudo service nginx reload```
+
+Your certificate is ready to serve. You can check that every thing is correct byan external service like <a href="https://www.geocerts.com/ssl_checker" target="_blank">geocerts</a>
 
-* /etc/yunohost/certs/YourCertifiedDomain/crt.pem
-* /etc/yunohost/certs/YourCertifiedDomain/key.pem