I always use a line from the centre cleat long enough to reach back to alongside the cockpit with an extremely large loop, maybe 4 or 5 feet across in the end ... get as near to a cleat on the pontoon or boat you're coming alongside and drop the loop over it ... sometimes you might need to use a boathook to do this. Use the same rope for the job each time and put some white tape round it where it needs to tie on the centre cleat so you don't need to worry about the right length every time you use it. Engine very slow ahead and tiller hard over and fixed with a handy loop of something, small stuff or a bungee, to keep the stern alongside .... she'll sit there all day like that giving you all the time you need to tie everything up.