In Adobe Photoshop, merging or flattening layers involves combining multiple layers into a single layer. Here are two common ways to achieve this:

Merging Layers:

Method 1: Merge Down or Merge Up

1. Select the Layers:

   • In the Layers panel, select the layers you want to merge. You can do this by holding down the Shift key and clicking on the desired layers.

2. Merge Down:

   • To merge the selected layers into one layer, right-click on any of the selected layers and choose "Merge Layers" or "Merge Down" from the context menu. Alternatively, you can use the keyboard shortcut Ctrl + E (Windows) or Cmd + E (Mac).

3. Merge Up:

   • If you want to merge layers above a specific layer, select the bottom layer and choose "Merge Up" from the context menu. This will merge the selected layers into the one directly above the bottom layer.

Method 2: Drag and Drop

1. Select the Layers:

   • Again, in the Layers panel, select the layers you want to merge.

2. Drag and Drop:

   • Drag the selected layers onto the layer you want to merge them into. This creates a similar effect as merging down, combining the selected layers into the one you dropped them onto.

Flattening Layers:

1. Flatten Image:

   • To flatten the entire image, merging all layers into a single background layer, go to the "Layer" menu and select "Flatten Image."

   or

   • Press Ctrl + Shift + E (Windows) or Cmd + Shift + E (Mac) to create a new layer containing a merged copy of all visible layers.

   • Note: Flattening is irreversible, so it's a good practice to keep a copy of your layers before flattening if you might need to make further edits.

Remember that merging or flattening layers removes the ability to edit them individually. Before doing so, consider saving a copy of your work or creating a duplicate of the layers you want to merge in case you need to preserve the original state for future adjustments.