Always attach an anchor to your replies. It's half-width '>>' plus a number (for example: >>1).
Literal
Reply [to-には] anchor [obj-を] definitely attach-please. Half-width [with-で] '>>' + number (example: >>1) is.
2channel-style forum instructions. アンカー is the standard jargon on Japanese BBS/2ch for a reply marker like '>>1' pointing to post #1. ~には on 回答 marks it as the target/context for the following instruction. 半角で identifies the character-width style (here half-width ASCII '>>'), as opposed to 全角 (full-width) which would render as '＞＞' with double-byte characters. で means 'by means of / in [that style].'