Find cofounders with the expertise and contacts you need, or at the very least advisors. There's a lot of legal complexity, you'll need to get FSA approval and do things like setup ringed-off accounts to hold client money.

Depending on your definition of small you might also need to figure out ways to reduce transactions fees which may mean taking on risk yourself, in which case you'll need to ensure you have suitable risk expertise on your team.

One way to make things easier might be to whitelabel an existing brokerage service as a step up to being a full broker.