There are a few ways.
Firstly you can travel as an independent referee. You would need to contact Daniel Curcio who is a member of this forum or check out his posts to see the links to where to apply. You will need to talk to your County FA so they can apply for international clearance on your behalf. If you search Facebook for Tournaments Abroad you should find the relevant page as well.
Secondly you can talk to your local Referees Association as they may be looking to take a group to a tournament (this is how I am travelling). They may have some sponsor money which could contribute towards the trip. In my case my RA have provided kit badged up with their badge and the name of the tournament. I have paid for my flights, my train tickets, the extra nights in the hotel (the tournament nights are paid for by the organisers) and my spending money. The RA made the application for international clearance on my behalf. We have 4 RAs in my County who are sending referees to tournaments in the close season. Most involve flying (Dubai, Spain, Portugal) but another has stayed within the UK and involves road trips to Bolton and Cardiff.
Thirdly if you are involved with your County FA's School of Excellence or Academy, they may send a group. The arrangements for funding, clearance, etc. are similar to the RA trip.