Where do you live?

Spray with a product containing atrazine. Here is an example...

That stuff kills just about everything except St Augustine and Centipede grasses. Follow the directions to the letter. I believe it recommends walking backwards so you don't have to walk through the spray.

Water deeply and infrequently. Deeply means 1 inch all at one time. The frequency depends on the current weather. With temps in the 80s, deep water once every 2 weeks.

Always mow St Augustine at the mower's highest setting.

Fertilize once at the end of May, once at the beginning of September, and once around Thanksgiving.

Thank you for your advice. I live in the Houston area with August temps hitting in the 100's at times and January temps dropping to the 20's for a day or 2. Currently my sprinklers are off because we are getting rain every day. I am waiting for it to dry out first and then I will try your advice. I am trying to upload pics of the criminal weed with no success. I will try again.

One blast of that herbicide will make a huge difference. The St Aug will come from everywhere and fill in. It doesn't work overnight. It takes 3 weeks to notice that all the weeds are gone and the grass has filled in.

Atrazine won’t hurt the crabgrass unfortunately. You’ll have to wait for frost to kill it, and then you can pull it out when it’s dead.

Id also recommend you don’t make the same mistake as me and use a preemergent in the spring. Otherwise the weeds will just all come back

Also dchall on a related note, what fertilizer do you recommend for the thanksgiving application?