It took us 14 months, starting at the age of three... Not wishing to sound gloomy here. We did great at first, but then hit regression and back to square one. My tips are:
Go straight to loo (don't use pottys as then you have another hurdle when you wish to get rid of the potty).
Get a special loo seat that has a flip down kid seat in the lid (B&Q sell these)
If needed, for example if you child has balance issues, build a large box step all the way around the loo (water proof mdf worked for us, do you know anyone handy with a saw / drill?)
Use bribes. All sweets were banned in our house, except a smartie / choc drop after anything went in the loo
Visual time tables in the loo and somewhere obvious like the fridge
Keep a spreadsheet noting when anything happens to help you identify a patten
Use a reward chart with an associated visual time table. Pic 1 child with legs crossed and worried look. Pic 2 walking to the loo - we put child with footprints in front going to the loo. Pic three sitting on the loo. Pic 4 something in the loo. And so on. We had to include a picture of mum putting tick in the reward chart at the end so DS could understand the reward chart concept
Do not use pull ups - this just made DS think he still had a nappy on.
We didn't even use pants for a while as the feeling of 'containment' made him think he still had a nappy on - so he went commando. We explained to nursery and they were fine. In the end we got some rather large baggy boxer shorts which were OK.
We also out fly stickers in the loo which he could aim at (make sure you put them right by the water line of wee goes everywhere)
Put Ping-Pong balls with faces drawn on for your child to aim at. When you hit them they spin about which is fun.
You will get there. It's a long road. Thank god we had wooden floors downstairs (and kept a baby gate up for the duration). We bulk brought antibacterial wipes and just had mega amounts of kitchen towel, cleaning products and sterilising solution to hand. In the end it just became part of life, all those accidents, and looking back I sometimes wonder how we had the patience. But what a relief once you're out the other side. Hang in there.