@S'mon has some experience and advice. This is some examples from other posts of how he goes about it:
"The general rule if in doubt is "divide number above 10 by 2" - so AC 25 becomes AC 17 (or 18 if that looks better). Use attribute bonus for saves, only give Proficiency if it looks really necessary. Hit Points should normally be increased +50%. Damage is trickier, but +50% generally seems to work likewise. Abilities that disable PCs for the duration should generally become save-each-round. Recalculate Attack Bonus based on Profiency (derived from CR), and Attribute bonus. "
"Converting damage and hit points - I started off doing roughly x1.5 damage and x1.5 hit points (while attack bonus is calculated off stat bonus + Proficiency bonus). This worked for a long time, but at very high levels gives excessive hit point totals, especially where AC is good, so for double-digit CR monsters it is often best to keep the PF hit points as listed."