Either before you apply it or after its glued,the same as you would the back of your self bow..Either the older members on here have forgotten,and newer members dont know,that Manny Pedrini,used to make Boo backed red oak bows and he trapped the boo and was happy with them..He also used Boo more narrow than the backs surface and created a crown,on the back...said they were strips from window shades.... Another Member{cant recall his handle}used to trap the back of Red Oak self bows by as much as 60% and said it helped eliminate the string follow associated with traditional styles,without way over building them...