You need to set routing options for aggregating your subnet, set router-id, the create the export filter, then export the filter to peer.
set routing-options aggregate route 1.2.3.0/24
set routing-options router-id 1.2.3.4
set policy-options policy-statement to-peer term export_prefix from protocol aggregate
set policy-options policy-statement to-peer term export_prefix from route-filter 1.2.3.0/24 exact
set policy-options policy-statement to-peer term export_prefix then accept
set policy-options policy-statement to-peer then reject
set protocols bgp group peer
set protocols bgp group peer type external
set protocols bgp group peer peer-as 1234
set protocols bgp group peer local-as 1234
set protocols bgp group peer neighbor 1.2.3.4 export to-peer
This will get you started.
You might need to set hold-time, import rules etc.