Go to The Tile Shop and get the acid based grout and tile cleaning liquid, used for stripping the old sealant and for removing grout haze. Use that and a scrub brush and elbow grease.
Then let it dry 2 days. Then use the Gold sealer. It will cost you some $. Both chemicals are expensive. But...
It takes carpet installers years to get the hang of seaming carpet I commend your willingness to DIY but the best I could tell ya if you’re adamant about doing yourself is YouTube videos.
I’ve done hardwood and tile flooring for 15+ years and wouldn’t seam carpet in my own house.