Here is a good instructional video produced by Park Tools. I will add some personal comments and suggestions further below.
In the video it was pointed out that the threads should have either an anti-seize product or grease applied. This is a very good idea as if you have ever encountered pedals that are extremely difficult to loosen and remove this the reason why as none was used when they were installed. If you find rgat you can’t loosen the pedals there some things you can try. My first recommendation is to try impact on rhe wrench. You can smack it with palm of your hand if you are tough enough to do so. You can use a soft hammer so as not to damage the wrench. You can also use a piece of wood to either place on the wrench handle to help protect it and use a regular steel hammer to smack the wood. You can use a board (such as a 2×4) as a hammer to smack the wrench handle. If you find the pedal threads don’t want to cooperate and turn to loosen you can try tightening it a bit more and then try loosening it. If you can’t budge the wrench to tighten it you can use impact. Just don’t try to turn it very far in tightening it. If you experience the threads being very tight and uncooperative as you try to unscrew it you may have to try using special penetrating oil such as WD-40. Even after trying that it may be a good idea and necessary to turn the threads both directions back and forth to carefully remove the pedal without doing damage to the threads. I would advise you to continue to use the penetrating oil frequently as you turn the threads back and forth as this will aid the penetrating oil to “penetrate” and do it’s job. There is always the possibility that a threading tap should be used to clean up the threads before a new pedal is installed in a crankarm that you had a difficult time removing the pedal. Hopefully you won’t encounter this problem, but if you do I think this advise will be helpful. Let’s all try to …
KEEP ON TRIKIN’