You never let them go out the door first. You come out, they follow. You are leading them not them leading you. Use a halti or similar to enforce this at first if you need to but you don't want to be relying on it long term.
Every time they lunge or pull, you stop dead. Pay no attention and don't start moving again until they stand quietly with you. This may mean stopping literally every step for an hour but if that is what it takes, that's what it takes. Combine this with a command. I just say stop pulling. The work doesn't matter as long as you do it the same every time. I didn't use food as a reward but you can do.
At the very beginning, I excused pulling until I'd let him tear round the field for an hour. There's no good starting teaching something new when they're high as a kite. Wear them out and then introduce the new training on the way home.
Depending on the dog, do this for a few days or a few weeks. Mine are labradors so they are people pleasers and naturally pick up new things quickly. Some more stubborn breeds might take longer.
Then start gradually introducing the concept that this is how we walk all the time. With Ddog1 I did it by taking her into town with me after the walk and stopping every time she pulled. They learn best when they're knackered. Otherwise it's like reasoning with a sugar high toddler. Nothing gets through.
Gradually extend this to being the way we walk at ALL times
Never use a harness or flexi lead. Harnesses just let them put their full weight behind it and drag you. Flexi leads, you get more lead by pulling. They're not daft. This teaches them the opposite of what you want them to learn.
If you need one because their recall is shaky, use two. The flexi is the safety line only. You set it slightly longer than the normal lead and use only the normal lead to direct them. I find chain leads better because they are heavier so the dog is more aware of them. Do not use the slip chains that tighten when pulled. They don't help and are dangerous with a dog who pulls. Just a normal chain lead with a clip for the collar. Use a proper collar with a buckle, not a clip like you get on pram straps as these can break apart if yanked hard enough.
Then when you get to the area where they are allowed to run or wander. Say a signal like go play and remove the chain lead in favour of the flexi.
Try and get them to return to you before the hit the end of the line and yank your arm off. When teaching recall never call them back for anything negative. Call them back, give treat or throw ball and release them again. When it's time to go home, just walk over and attach the regular lead without a command or signal. You don't want them to associate returning to you when called with the end of the fun.