Other than putting the slip lead on the right way around it’s also really important to have the lead in the right place!
If you are right handed, the loop for the slip lead should be held in the right hand for security and the left hand used to guid the dog by holding the lead vertically from the dog whilst your arm/hand is in line with your left knee. The lead should be around the dogs neck, very high up to the ears - ideally vertically to your left hand!
Using a short lead from left hand to dog, encourage heel work by giving the “heel” command only when the dog is in the correct position (head by your left knee). Use the dogs name to get attention and correct any pulling forward with a “no” “ah, ah” or whichever command you use. Once back in position praise and give the heel command. Change direction often to start with, it will keep you dog on its paws and completely focussed on you. 5 minutes practice, 3 times a day for two weeks and you should have a dog that walks to heel beautifully without the need for a harness…..