Firstly i assume that you already have Xmrig installed on your computer, if not you can get it at 
https://xmrig.com/ or at their  github to get latest release 
https://github.com/xmrig/xmrig/releases.
If you are using the  
config file you can do like this: 
Open up config file with lets say notepad and scroll down to pool configuration section.
{
            "algo": null,
            "coin": "null",
            "url": "foundation.biblepay.org:3001",
            "user": "Your_monero_adress.your_preferred_workername",
            "pass": "Your_biblepay_adress",
            "rig-id": null,
            "nicehash": false,
            "keepalive": false,
            "enabled": true,
            "tls": false,
            "tls-fingerprint": null,
            "daemon": false,
            "socks5": null,
            "self-select": null
        }Just copy and paste your pool configuration and add a comma  in the end.
Should look like this when you have failover pool setup:
{
            "algo": null,
            "coin": "null",
            "url": "foundation.biblepay.org:3001",
            "user": "Your_monero_adress.your_preferred_workername",
            "pass": "Your_biblepay_adress",
            "rig-id": null,
            "nicehash": false,
            "keepalive": false,
            "enabled": true,
            "tls": false,
            "tls-fingerprint": null,
            "daemon": false,
            "socks5": null,
            "self-select": null
        },
        {
            "algo": null,
            "coin": null,
            "url": "bbp.miningpool.fun:3001",
            "user": "Your_monero_adress.your_preferred_workername",
            "pass": "Your_biblepay_adress",
            "rig-id": null,
            "nicehash": false,
            "keepalive": false,
            "enabled": true,
            "tls": false,
            "tls-fingerprint": null,
            "daemon": false,
            "socks5": null,
            "self-select": null
             }First pool configuration from top will be your primary pool and the one below is  your failover pool, you can set more failover pools if they are available. 
If you are using the command line or made a batch file it would look like this: 
xmrig.exe --donate-level 1 -o bbp.miningpool.fun:3001 -u Your_monero_adress.your_preferred_workername -p Your_biblepay_adress -k -o foundation.biblepay.org:3001 -u Your_monero_adress.your_preferred_workername -p Your_biblepay_adress -k