Learning all the rafsi and the permissable consonant pairs in order to know how to make lujvo seems like a lot to learn, and it would be if you had to just memorize it. I think it's easy though, if you just learn it through learning lujvo. For instance, if you know the lujvo "lobypli" (lojbo zei pilno, lojbanic user, a user/speaker of Lojban), then you already know two rafsi, "lob" and "pli", and you know that you need a "y" hyphen between b/p. If you know "badgau" (bandu zei gasnu, defense do, defend), then you know another two rafsi, "bad" and "gau", and that you don't need a hyphen between b/g. You already have examples of permissable consonant pairs from all the CVCCV gismu you know-- for instance you know "gismu", so you know that you don't need a hyphen for s/m. In this way I think it's easy to accumulate the knowledge necessary for lujvo making from direct experience with the language, without the need for memorization. You can think back on the model provided by the words that you know, which are reliable guides because of Lojban's regularity.
The same is true with the shapes of lujvo. In fact I recommend learning lujvo shapes one at a time. The shape CCVCCV has long been suggested as a simple shape to begin with, because it never requires a hyphen. The same is true of the shape CV'VCCV (as in fu'ivla or gi'uste). The shape CV'Vr/nCV'V (as in ta'irva'u or ro'inre'o) is almost as simple, since it always requires an r-hyphen (or an n-hyphen when the second rafsi starts with r). And don't be afraid of the easiest shape of all, unreduced lujvo: cninyprenu, fukpyvalsi, lojbypilno-- just replace the last vowel of the first gismu with an "y" and you're good to go-- maximally easy for your seltavla to recognize, as well. So be creative! Lojban needs the vocabulary, so take your place in history. Just please mark your creativity with "za'e", so you don't send anyone on a wild goose chase to the dictionary. :) mu'o mi'e .selkik.
