Question: How are snoRNAs guided to a spot on an RNA?
About the article "RNA", section "In RNA processing".

Candidate passages:
Many RNAs are involved in modifying other RNAs.
Introns are spliced out of pre-mRNA by spliceosomes, which contain several small nuclear RNAs (snRNA), or the introns can be ribozymes that are spliced by themselves.RNA can also be altered by having its nucleotides modified to other nucleotides than A, C, G and U.
In eukaryotes, modifications of RNA nucleotides are in general directed by small nucleolar RNAs (snoRNA; 60-300 nt), found in the nucleolus and cajal bodies.
snoRNAs associate with enzymes and guide them to a spot on an RNA by basepairing to that RNA.
These enzymes then perform the nucleotide modification.
rRNAs and tRNAs are extensively modified, but snRNAs and mRNAs can also be the target of base modification.
RNA can also be methylated.
Small nucleolar RNAs (snoRNAs) are a class of small RNA molecules that primarily guide chemical modifications of other RNAs, mainly ribosomal RNAs, transfer RNAs and small nuclear RNAs.
There are two main classes of snoRNA, the C/D box snoRNAs, which are associated with methylation, and the H/ACA box snoRNAs, which are associated with pseudouridylation.
SnoRNAs are commonly referred to as guide RNAs but should not be confused with the guide RNAs that direct RNA editing in trypanosomes. 
An unusual guide snoRNA U85 that functions in both 2'-O-ribose methylation and pseudouridylation of small nuclear RNA (snRNA) U5 has been identified.
This composite snoRNA contains both C/D and H/ACA box domains and associates with the proteins specific to each class of snoRNA (fibrillaring and Gar1p, respectively).
More composite snoRNAs have now been characterised.
These composite snoRNAs have been found to accumulate in a subnuclear organelle called the Cajal body and are referred to as Cajal body-specific RNAs.
This is in contrast to the majority of C/D box or H/ACA box snoRNAs, which localise to the nucleolus.
These Cajal body specific RNAs are proposed to be involved in the modification of RNA polymerase II transcribed spliceosomal RNAs U1, U2, U4, U5 and U12.
Not all snoRNAs that have been localised to Cajal bodies are composite C/D and H/ACA box snoRNAs. 
Non-coding RNAs (ncRNA) are RNA molecules that have a function but are not translated into proteins.
Small nucleolar RNAs (snoRNAs), one of the largest classes of ncRNA, are further subdivided into the two major C/D and H/ACA snoRNA families.
snoRNA serve as guide RNAs for 2'-O-methylation and pseudouridylation of specific nucleotides and indicate the site of modification by direct base pairing with the target RNA.
The majority of these snoRNAs are responsible for the post-transcriptional modification of ribosomal RNAs (rRNA) and in some cases of small nuclear RNAs (sRNAs).
These post-transcriptional modifications are crucial for rRNA processing, stability and maturation.
The H/ACA snoRNAs that guide pseudouridylation in Trypanosomes consist of a single-hairpin followed by an AGA-box.
These H/ACA snoRNA contain a pseudouridylation pocket that guides the H/ACA snoRNA to the correct region of its target rRNA.
The pseudouridylation pocket ensures modification of the correct uridine, as it contains two short sequences that are complementary to the target RNA.
In addition it is able to basepair with the regions flanking the uridine in need of modification.
Trypanosomal snoRNAs differ in both sequence and structure to eukaryotic snoRNAs as they contain one stem loop and an AGA-box at the 3' end.
Despite these differences, trypanosomal snoRNAs function identically to eukaryotic snoRNAs and since the discovery of single stem H/ACA snoRNAs in Trypanosomes, similar single hairpin ncRNAs have been discovered in Archea and Euglena.
The table below summarizes the H/ACA snoRNAs and the corresponding target sites identified in Trypanosomes.
H/ACA snoRNAs identified in Trypanosomes  
Small nucleolar RNA sR8 is a non-coding RNA belonging to the C/D box class of snoRNAs.
sR8, along with other C/D box snoRNAs, performs 2′-"O"-methylation of ribose on a target strand of ribosomal RNA.
Targeting is achieved through the C and D box components, which are short sections of conserved sequences, as well as C'/D' boxes (internal equivalents of the C and D boxes).
These sequences base-pair with nucleotides in the target rRNA to direct their methylation.
sR8 RNA operates alongside protein components in a ribonucleoprotein (RNP) complex.
sR8 is able to guide the methylation of targets found within structured stem-loops, which implies that a component of the complex is able to unwind such structures.