Hi Nevon,
in a basic setup:
-you do this inbound for any route with path prepend if you connect to multiple ASes (you add the same AS a few times, so that you have a longer path for that route over one of your routers)
-you can do outbound traffic steering with local preference
-if you connect to multiple routers from a single AS on the other side you can use MED to steer inbound traffic
There is more to it, the details are quite complicated, so I suggest you should read some docs.
I find one of the best free resources on BGP is from Juniper:
https://www.juniper.net/documentation/u ... index.html
BR
W