Radicals are a way of classifying kanji according to a number of common elements. Use of Radicals as a strategy means knowing the meaning of the radical and relating that meaning to the meaning of the kanji was a whole.
1 I group the kanji with other kanji containing the same radical
e.g.1 痛い (いたい painful), 疲れる (つかれる to tire), 病気 (びょうき sick) all have the ‘sickness’ radical meaning
e.g.2 駅 (えき railway station), 駐車場 (ちゅうしゃじょう car park), both have the ‘horse’ radical (馬 うま). Transport used to be by horse.
e.g.3 池 （いけ pond） 海 （うみ sea） 注ぐ（そそぐpour）all have the ‘water’ radical.
2 I remember the radical first and it helps me remember the kanji
e.g.1 食べる (たべる to eat): ご飯（ごはん cooked rice or meal）and 飲む (のむ to drink) are both related to eating so you expect them to have the same radical.
e.g.2 言う (いう to say) and 話す (はなす speak) both use words coming out of a mouth
e.g.3 茶 (ちゃ tea): has the grass or plant radical at the top.
3 I associate the kanji with the meaning of the radical
e.g.1 送る (おくる to send) has the movement radical. Sending something somewhere involves movement.
e.g.2 机 (つくえ desk) has the tree radical. The desk may be made of wood.
e.g.3 結婚 (けっこん marriage) has the ‘thread’ radical (糸 いと). Getting married is sometimes referred to as ‘tying the knot’.