Determine a suitable fee that each of the participants can easily pay. Or you can ask each of the participants to pay on a donation suggested basis if setting a particular fee does not seem to be a good idea.
Visit a few bars and make them aware of your plan to organise a fundraiser pub crawl and ask the manager at each of the bars you visit to make a donation in terms of drinks. The participants of the pub crawl can then purchase those drinks at a certain cost.
You would want to prevent random people, with no interest in a public cause, from fundraising for their own use. The best way to accomplish this is to invite clearly distinguishable participants to the fundraiser.
Decide which bars you would include in the pub crawl and schedule the event. Determine the route and pick an itinerary.
Before the scheduled date, contact all the bars on the pub crawl route and ensure that the respective bar managers are still onboard.
Pick a colour theme for the fundraiser. If you plan to raise funds for a medical charity, it would be a good idea to ask the participants to wear nurse and doctor costumes. Similarly, if you are raising funds to help fight cancer, the theme you choose should be pink in colour.
At least a week ahead of the fundraiser, send out invitations to all the people you plan on inviting. Along with the invitation, include a route map as well.
It would be a good idea to give something back to all the participants. Get some shirts printed for this purpose.