Doof the Tumbleweed is the first of the Bank Robbery sidequests. You cannot invest in the Banks and be a bank robber, you have to choose one or the other.
In order to start this quest, talk to Slim
He can be found in the building across the road from the bank. When told, head into the lobby of the bank before heading into the Staff area of the bank, deal with the Security guard before Slim cracks the bank vault.
Once he does, the Sheriff may be alerted. If he is, don’t try and juke it out with the Sheriff, you will lose and raise your bounty level. Try and just run away down Highway 2 to the little shelter that Slim asks you to go.
Once you have finished this quest, you may get a wanted level or a bounty level. Where possible, do this quest before taking the Bring on Taylor quest so that you get the free wanted level clear.