Boilers are quite complicated to install so I think the best advice I can give is to call a specialist to install it for you. When I needed my boiler replaced, I contacted this company http://www.jameswinney.com/boiler-installers-london/ who offered boiler replacement & installation in South London. They dealt with everything for me.
Unless you have some background working with boilers, then seeking professional help is probably the best thing to do. You could try to follow an online guide and complete the installation yourself, but if you do it wrong, the mistakes could be costly, as most boilers don’t come cheap. There is also a chance that the company you buy the boiler from might offer free installation. If you’re looking to save some cash, you should ask the company when you buy it to see if they offer free installation.