If the wheel is upright, meaning the drive side is on the right, you have to turn the nipples counter clockwise to tighten. Imagine you have the tire, tube and liner stripped off. You're tightening the nipple from the outside of the rim using a flathead screw driver. Turning the screwdriver clockwise to tighten means that the nipple is turning counter clockwise on the other side, inside the rim.

Start by squeezing each spoke together to check for any which are noticeably looser than the other. Keep in mind that the drive side on the rear wheel requires more tension than the non drive side.

Generally, I like to begin by applying an even amount of tension onto most of the spokes while looking for outliers which are particularly loose. Twist one of the spokes first to be doubly sure you are tightening or loosening as intended. Sometimes, when a rim is bent in a specific area, you will have to sacrifice spoke tension for rim trueness by loosening the spoke(s) in that area to keep the rim true. Try to find the right balance.

After an even amount of tension is applied, now, look for areas of the wheel which still appear to be bent. You want to tighten the spokes on the side you wish to move the rim towards - (ie. If the wheel is bent to the left, i wish to move it to the right to centre, meaning you tighten the spokes on the right side, and perhaps loosen a bit on the left side)

if any of the spokes are stiff, or pop back into place when you turn, you run the risk of snapping the thread if you try to keep twisting. Beware.

Lastly, once you are reasonably satisfied with your balance between spoke tension and rim true, place the rim onto the ground, and holding onto the opposite ends of the rim, gently press down into the ground. Go around the entire rim. You should hear the spokes gently clicking as they set in place. Go around the entire diameter and repeat on the other side. Don't press too hard. But don't be alarmed if you don't hear any clicking: the spokes are likely already set. You don't want this to happen once you put your entire weight onto the wheel once its mounted on the bike.

This process takes a lot of prsctise and patience to get right. Practice, and don't get frustrated if its not working. Watch another YouTube video, take a brea and try again. The rim doesn't need to be perfectly true if youre struggling.