Crawl under the rhododendron and look upward into its structure. Reach up with a pair of hand-held pruners and remove any dead branches. Remove excessive cross-branches or branches that appear damaged or weak. Prune rhododendron branches flush with the main trunk or trunks; never leave stubs.
Decide how many of the bottom limbs you wish to remove. Consider the size and structure of the rhododendron -- a shorter plant may be more attractive with fewer limbs removed, depending on the overall landscaping. Use hand-held pruners to cut small lower limbs flush with the trunk of the plant, stopping frequently to check that the plant is trimmed evenly.
Watch the plant after you've cut out the lower branches -- rhododendrons sometimes resprout from the bottom, near the crown. Remove any errant sprouts that appear, cutting them flush with the plant's trunk.