It is actually quite simple of a setup, even if you didn't configure BGP before:
Set up Fortigate (FGT) WAN interfaces with relevant /30 IPs and verify the links and IPs work fine - pinging point-to-point, loading line with laptop if it is a new line.
Configure BGP on the FGT
Regarding BGP - as this is a small (/29) pool, it means you are getting Provider Assigned (PA) IPs, not your own AS numbered, so you will have to set on your side Private AS, say 65001. Also, you should ask your provider whether you need to add AS Path prepends for the /29 you advertise via Backup line or they will do this backup/main line manipulation on their side. If they say you should advertise /29 with prepends over the Backup line, this will add route-map config on your FGT. If not - it is the most basic set up at all.
E.g. let's say Main Line IP is 126.96.36.199/30 and is set on port1 in FGT, and Backup line is 188.8.131.52/30 on the port2 in FGT, AS number of your ISP is AS 1680, and you advertise 10.10.10.0/29 which is configured as directly connected on the FGT, then :
config sys int
set ip 184.108.40.206/30
set ip 220.127.116.11/30
1. Route-map to add prepends
config router route-map
edit 1set set-aspath "65001 65001"
2. BGP neighboring
config router bgp
set as 65001
edit "18.104.22.168"set remote-as 1680set weight 10
edit "22.214.171.124"set remote-as 1680set route-map-out "prepend-out"
config redistribute "connected"set status enable