What do arranging and assigning mean in sequence annotation?
Word "arranged" means simply alignment of sequence based on overlapping regions in two sequences. For example:

Seq1: AGTGTCCGTCGTAAGTCA

Seq2:GTCGTAAGTCATCGAGATACC


leading to:

Seq12: AGTGTCCGTCGTAAGTCATCGAGATACC


Maybe he used the word "arrange" as overlapping determines the direction of resultant sequence (for which word alignment is sufficient)

Assignment: Is simply dividing the genome's sequence down as per chromosomes of the species. Like for example, if it's human genome sequence and first chromosome is 2,50,000bp long (just assumption - I don't remember exact figure), then first 2,50,000 nucleotides in the sequence will be "assigned" to chromosome 1. From 2,50,001 onwards to chromosome 2 and so on..