The apron rod bends.
It’s important that the apron rod stays straight. Slip off excess lashing on the rod as shown. You only want lashing on the apron rod to be as wide as the warp that will be tied on. I’ve seen huge dowels to try to prevent bending. That doesn’t work. It will be easy to slip the lashing onto the rod again for wider warps, so don’t worry about that.
Check the warp tension:
Close your eyes (that’s important; it helps you to concentrate), and use the flat part of your fingers to pat gently across the warp. All the threads should feel the same. If they’re too tight or too loose, they should be re-tensioned.
Don’t agonize. That’s important, too. Unless a bundle of warps feels definitely softer or definitely tighter compared to the others, it is probably just right.
I like to tie on with surgeon’s knots. If you need to adjust bundles, just tug one of the tails of the knot to open or loosen it. There is no need to untie, then retie the knot. To tighten a warp bundle, brace the heels of both hands on the apron rod and pull the tails of the knot to cinch the warps tighter.
Complete the surgeon’s knot:
When you’re satisfied that the tension is even, take the tails of the knots and tie them together to complete the final part of the knot. In other words, each knot will consist of the beginning parts of a surgeon’s knot with the final part of the knot on top of it. See my eBook on knots and of course, Weaving for Beginners for how to tie this and other knots for weavers.