First off, you do have a barrel adjuster where the cable enters the derailleur, right? That is for tightening. Make sure it is screwed all the way in first, then secure the cable, then start unscrewing to tighten.

More tips:

It is possible to buy inline barrel adjusters that can give you more cable-tightening ability, even while riding.

It is possible to buy 'third-hand' bicycle tools that hold cable under tension while you tighten the bolt.

But, you shouldn't need to buy specialty products to install a derailleur cable.

When you secure the cable initially, try to get it as tight as you can to begin with. Lock some vise grips on the cable and pull with one hand while tightening the pinch bolt with the other.

Also, use the limit screw to push the derailleur in maybe half a cog-spacing, then attach the cable, then loosen the limit screw again to let the derailleur back out, that should add some more tension as well. You'll have to start from scratch with adjusting your limit screws, but it's not a bad idea to do that anyways when replacing cables.