AS @Julien suggested painters tape and super glue is very good. However if your project gets more than 12" square that takes a lot of tape and super glue. I have an SO3 XXL with a supplemental spoilboard that has 2" spacing for 1/4-20 tee nuts. I use cam clamps to hold down large projects. The cam clamps are easy to make but do require a way to anchor them. If you have a newer machine with the hybrid table not everything will line up with the t-tracks for clamping.
I use a long L bracket that mostly stays on my spoilboard all the time. This gives me a good base to line up projects square and gives me an anchor for cam clamps on the opposite side.
In the file are two long L brackets and several cam clamps. The L brackets were designed around a 2" spacing for my spoilboard but youi can simply redesign and move the holes around to match your machine spacing.
The advantage of the cam clamps is they are flat and even if you go over the edge of your project they are made of wood and can easily be replaced.
Warning: my file uses bottom of material and center for the origin. So be sure to change the job setup to your preferences. I use the bottom of material to prevent my spoilboard from being cut up prematurely. Additionally I like to use the center for most jobs because I just like that. Also measure your material and enter that as well. My material was what I had on hand but the thing I do is the large recess hole is always .5" from the bottom of the material so all the cam clamps I have can use the same length bolts without going through to the base board and causing material height problems.
Here is a reply i made a while ago about what I like to do. I have since started using the L bracket as well but depending on the situation I really like to run an operation to have the CNC create holes that match my threaded inserts in my table and then use those holes to hold down the part for the actual machining.
I’m a big fan of the super glue and painters tape method. If you ever choose to use that keep in mind that as you get a larger workpiece you don’t need to cover the entire surface. If carefull you can lay a few strips and carefully align your project not to glue it to the table. I wouldn’t wallpaper the entire surface of a larger project and not only because it gets expensive in materials but also because you may break your workpiece trying to lift it off the bed of the machine… On a 32" x 20" workpiece i had used 6 strips of tape and tought i would need a crowbar to get it off afterwards. Was very concerned of snapping the board at the glue seams (5 boards glued together). After 2 days of milling that would have been very disapointing.