Few-mode graded-index optical fibers are being increasingly relevant in spatial division multiplexing. Likewise, multi-region binary phase plates are fundamental elements for some mode multiplexing schemes in such optical fibers. In this work, we propose a coupling configuration for mode multiplexing based on collimating-focusing graded-index lenses together with binary phase plates, and calculate, in a fully analytical and quasi-analytical way, the theoretical conversion efficiencies and crosstalks between the fields generated by such plates and the Laguerre-Gaussian modes. These modes describe, directly or by a linear combination of them, the first optical modes of many graded-index optical fibers. The results obtained provide both the illumination conditions of the plates and their optical characteristics. The fabrication of the plates is made by using ion-exchange technology and their optical characterization by beam profilometry. The experimental conversion efficiencies are in agreement with the theoretical results.