With PEX, you really shouldn't need a manifold just to put in a softener. Assuming you're installing a softener into an existing system, you will need to find the pipe just downstream of the water meter before the first tee. You will want to put in two tees and a bypass valve between the tees so you can turn off the water softener and bypass it if necessary.
Also, have shutoff valves on the supply and return side of the water softener. This setup is called a "3 valve bypass" if you want to do more research on it. Best practice would be to use quarter-turn ball valves instead of of gate valves shown in the diagram. Beyond that, it should be fairly simple. Good luck.