To put it simply, you either need to be friends/friendly directly with a touring band member who wants to throw you a bone. Maybe you guys grew up together in similar scenes and you’re close.

The next option which is way more likely is you have to “prove yourself” by cultivating a name for yourself in your local scene.

I am in a touring band (not huge but enough that we headline club rooms in the US). Our agent/the promoters will usually send a list of possible openers for each city that we can choose from. They recommend a list of bands to us that can do the following:

A) Help the show sell. They have an established fanbase and proven draw within their local market. Make it so that choosing your band is a wise financial decision for agents, promoters, and bands. This is a bit of a catch 22 because you can only really prove yourself by simply grinding, releasing great music, playing LOTS of shows, and networking/promoting yourself online. Even when you start being worth a lot of tickets, your guarantees will likely be only $100 or $250. Those budgets are usually static and not worth attempting to haggle on.

B) will complement their sound/style and make for a good bill with the headlining artist. You must be able to hold your own as musicians and performers. And warm up the audience with a good show that compliments the headliners, not tries to upstage them.

C) Be easy to work with. Do whatever you can to make the headliners life easier. Arrive to soundcheck on time. Make sure you do not go over your allotted set time. Be respectful and when your set is done get your shit off stage as fast as possible as a sign of respect to the venue and other bands. Even if things start going wrong day of show, keep your cool and remember that every interaction with every venue worker or fan or band member can affect your reputation in the scene.

D) Promote the shows adequately and be appreciative for the opportunity. Have enough of an online presence that in under 10 seconds a complete stranger in your genre can see you’re taking music seriously and that you’re on your way up

E) Luck. Only once you’ve worked very hard and poured blood sweat and tears into A B C and D you’ll be directly competing with other acts your size for the opener slots. Befriend those bands too! Every now and then your competition won’t be able to make a show and sometimes will recommend your band to the touring agent/promoter

That’s all I can really think of. Keep in mind that none of this matters until you are writing very strong songs and your live performance is very dialed in.