If a Newtonian does not have a center mark for the primary mirror:

If a reflecting telescope doesn't have a center mark on the primary mirror, there are several ways to align the secondary mirror to the center.

Either by estimating the center. This can be done using the three mirror mounts attached to the edge that hold the primary mirror in the primary mirror mount. However, this would be very inaccurate.

A Barlow lens projects a larger area. To do this, the alignment laser is inserted into a Barlow lens (Barlowed laser), allowing the center to be estimated more accurately. The stronger the Barlow lens, the larger the area.

There are alignment lasers that project a circular pattern onto the primary mirror. This allows for the most accurate estimation of the center.

Or you can remove the primary mirror with the mirror cell, measure the center with a ruler, and glue a ring reinforcement there. But be careful: don't touch the mirror during this process. Apply the mark carefully, preferably wearing woolen gloves.