Season ya meat pretty heavily. When cooking in the bag the juices will accumulate and "wash" some of the seasoning off later. Don't coat it on like a rub or get weird with the salt just season it well. Like a little more than normal but not by much.

Place the leg(s) in the plastic bag with the butter (and garlic if you have it). I tend to use quart/gallon sized bags depending on the need. Use the immersion technique to seal and make the bag air tight(google it - it's easy) or vacuum seal.

Via your sous vide or the stove and a thermometer - heat up a large pot of water to a constant 145° F.

Place the bagged mostly air tight meat in the water. You can add it before the water is up to temp. It doesn't matter. Make sure the bagged meat is under the water - use something to hold it under if needed. If sealed properly it shouldn't float.

Leave the bag in there cooking for 20 hrs. Yes 20.

Snag bag and dump out juices or save them if you wish to make a gravy or sumn.

Slap em in the oven or an air fryer on 400° for about 10 min. This is just to get the skin crispy. Don't over do it.

Eat it. It's the best turkey leg you've ever had. C'est bon.