Many ways.. common is by adding riboflavin or chitosan

Collagen by itself is not UV-crosslinkable. You can however modify it such that it does become photo-crosslinkable. Have a look for instance at the following paper or search for other papers using GelMA:

http://www.ncbi.nlm.nih.gov/pubmed/26414409

For the actual photo crosslinking, you add a photo-initiator like irgacure or riboflavin (which actually works with blue light instead of UV). The photo-initiator forms the radicals which trigger the crosslinking of the GelMA  

Dear Burak,

You can modify the collagen by introducing methacrylate group into the collagen. For the methacrylation, you can use methacrylic anhydride in the presence of base, such as DIPEA., or you can use methacrylic acid in the presence of NHS and EDC.

The methacrylated collagen can be crosslinked using UV light in the presence of photoinitiator.

You can read the following paper:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4243547/

and

http://pubs.acs.org/doi/pdf/10.1021/bm0257412