If the Bahtinov mask was made for the Seestar then it should fit into the lens opening in the same way as the orange solar filter.

But before you place it over the lens, do two things. First, go into the Seestar tutorials by tapping on the “Tutorial” icon in the upper right corner of the Home screen.

Then scroll down to the section titled “Turn On Manual Focus”. Read and follow the instructions in the tutorial to turn on manual focus.

Then read the tutorial section titled “Find & Shoot Objects” and follow the instructions to find a field of view that contains one or more bright stars more or less centered in the field of view.

Then tap the AF icon at the lower right side of the screen. Once auto focus says that it has finished, check on the left, lower side of the screen. You should see the “Manual Focus” panel. Take note of value displayed there. This is the proper focus setting that the Seestar has determine with it's autofocus algorithm. Moon, Solar, Planetary, and Scenery each may have a different focus point.

Next, carefully insert the Bahtinov focus mask in front of the Seestar lens and observe that now the individual stars shown on your phone or tablet will appear to have an X across them. The Bahtinov focus mask causes diffraction spikes to form from the star images. In addition there will be a diffraction spike that splits the X somewhere.

The goal is to center the diffraction spike so it crosses exactly in the center of the X. If if it doesn't, use the manual focus panel buttons to adjust the focus so that the spike is perfectly centered across the X.— Murray Foster 2024/06/25 10:59

Finally, remove the mask! I know a guy who has forgetten to do that twice (me).

It's actually easy, albeit a manual process. GoTo your target and then select SkyAtlas.

The blue rectangle is where you're pointed.

Drag the red rectangle to where you want your second mosaic image and then GoTo (overlap at least 10% but 25% is safer).

Take a screen grab or make a drawing so you can keep track as you build the mosaic.

Rinse and repeat.

Siril's Go Register and PixInsight Star Registration processes will help you put the mosaic together.