Installing and adjusting fenders takes some patience and is definitely the most complex part of the tricycle assembly. The fenders will require some bending of the struts to make everything line up and fit correctly. This is common for fender installations on bicycles and tricycles. The following procedure will get them assembled and adjusted the easiest.
1. Mount the fender struts to the tricycle frame with the 6 philips bolts and tighten them all down.
2. On the middle strut install the bracing hardware that secures it to the basket. The bolt goes through the strut first, then through all the black spacers, then the slot in the basket, with the nut on the inside. Pushing the fender strut in position so that the hardware lines up may be necessary. Keep this hardware loose.
3. Install the plastic fender strips on the middle strut first. Note that at one end of the fender strip, the hole is closer to the edge than at the other. Install the strip so that the hole closer to the edge is on the rearmost fender strut. Keep the hardware loose until all 3 points have been installed.
4. With all the hardware loose, it is possible to push/bend the struts so that the arch of the fender matches the curvature of the tire (mount the wheel first). Depending on your situation, you will need to adjust the position of the center strut first, before doing the front and/or rear.
Don't be afraid to push/bend the struts, this is expected in pretty much any fender set and the material can take the forces. You will most likely need to push the rear strut upwards to create a larger curve for the rear half of the plastic. Pushing up on a strut will create a larger curve and gap between fender and tire while pushing down will reduce the curve or gap.
5. Tighten all the hardware down. The curvature should not have changed, but double-check everything one more time.
6. Check the fender alignment in the vertical plane by looking from the top down. You may need to bend the front and/or rear struts to create proper clearance between in and the tire. Once this is done the fenders unless bumped or bent will remain in place.