As usual, the details are as important...
There are two ways to get thru San Luis Potosi up to Matehuala... the old way and the newer way. Take the latter.
Leaving Lakeside, and on the Chapala highway to Guad, take the Macrolibramiento 15D right, signed as to Zapotlanjo. This gets you heading north on 80D and skirts Guadalajara. Later after going through/around Lagos de Marino and when approaching Villa de Arriaga, go straight... not to the right. The sign at the intersection will say SLP 20 and Saltillo/San Luis Potosi straight. Don't go right which says 80D Queretaro. This road (straight) skirts SLP to the west/nw, becomes highway 49D and exits out north of SLP onto highway 57 north to Mathuala. For many, Mathuala is a good place to spend the night before heading to Laredo.
From Mathuala stay on 57 north to the outskirts of Saltillo. Take 40D right towards Monterrey; just prior to Monterrey, and at a toll booth, be in the right lane only to take the Periferico Nuevo Laredo Cuota (40). This skirts Monterrey to the west/north. At the north side one will take highway 85 north to Nuevo Laredo.
As you approach N. Laredo others have suggested taking the Colombia crossing. It is well to the west of Laredo and is signed off of 85 well. That crossing is easy but, crossing the border going north, I find the 'normal' N. Laredo crossing just fine... they separate trucks from autos. This route will dump you off right downtown Laredo Tx on I35 and not out in the boonies. Your choice of course.
P.S. This route uses 57 and NOT 54. Also, there is no 54D as "D" designates a toll road/cuota. 54 us not a toll. Also, you surely do NOT want to take highway 54 out of the north side of Guadalajara to Zacatecas! Slow, slow 2-lane going through curves uphill behind trucks and buses.