Searching the net to find out if it’s possible to make a cable without an RJ45 crimp tool yielded lots of people saying it’s not practical.
If you’ve only got a couple of ends to fit and don’t mind wasting a connector (or two) it’s not that bad.
Just splay the inner cores of the cable in the correct order and cut straight across the ends. Then feed the cores into the plug (making sure they go down to the ends of the correct holes). While keeping the wires pushed firmly into the plug, push down each metal piece in turn with a small flat-blade screwdriver.
Finally, give the cable a half-hearted tug to make sure they’re all fairly firmly stuck.
Obviously a crimp tool would give a much more reliable result, but at a push…