But sometimes you need more words than your first draft produced. You have a contract that demands a minimum of 75,000 or 90,000 or 100,000 words, and you're short, not by a few hundred, but by thousands. Or your editor says the story is "thin" here, or needs "beefing up" there. Sometimes a story is thin in one place, incident, character, or plot level, while being bloated somewhere else. Recognizing the skimpy places and knowing how to fix them is a complementary skill to knowing when and how to wield the slice-and-dice tools.
Adding words competently means enriching the story, not padding it. If it's skimpy, with its skeleton exposed, you want to add muscle, not fat. So the first step is to look at your story's skeleton. Is it all there? Particularly in novels, one cause of insufficient length is a missing part of the story--an incomplete subplot, a POV or secondary character without sufficient motivation, failure to connect the "head" to the "tail" with enough "stuff", even in the skeleton stage, to support the size body you want. Is it a big enough skeleton for the words you need? A short-story plot-skeleton won't support a 100,000 word novel. If you're dealing with a partial or stunted skeleton, you'll have to do serious surgery to give yourself the logical framework on which to hang more muscle-words.
But suppose you have the right size skeleton and all the parts are there....you're just short on words. Look at your skeleton, and where the meat on the bones fits...do you have equal meat all over, or do you have a thin place? Look at each important character--do you have the same levels of motivation for those at the same level of importance? If you've established that Jim, Toni, and Meg (members of team of four) are all drawing motivation from (example) childhood events, innate talents, family relationships, and pressure from their bosses, but Rob is drawing motivation only from one childhood event--then you can add words and enrich the story by revealing Rob's other motivational sources. In a novel, characters should be motivated by different things at different times (as we are) and have conflicting motivations in various combinations.
Suppose you've got the right size skeleton and you've developed characters at the same level about equally, but you still need more words. Look at the conflicts. Most scenes of conflict (arguments, fights) can be enriched and lengthened by revealing more of the characters' internal conflicts during the external one...and (if you haven't) some of the internal history that leads to this conflict. Most rows include both present and past causes, and those causes are both rational and irrational. The argument over whether to buy the front-loading or top-loading washer isn't just about which is more "green" or whether "green" should trump convenience and back pain...it's also about the power balance between the people arguing, about the need to be heard, about old fights lost and won, and so on.
Double-check your transitions. Transitions come in many forms: temporal, spatial, cultural, emotional...each risking readers coming unglued from your story. Readers must not be confused accidentally (mystery writers need to confuse readers but not by accident...) Small additions to some transitions can add a few words (each) while enriching the reader's experience. "Two weeks later, they reached the coast" does keep readers oriented, but "Two weeks later, after delivering the children to camp, the dogs to the kennel, and a final exhausting three-day drive during which they barely spoke, Jim and Allison reached the coast and checked into the condo they'd rented" gives the reader quite a bit to chew on...preparation for the loud argument they have as soon as they lock the door behind them about the discovery, in the previous chapter, that Jim had seen Allison kissing their daughter's soccer coach and she said it was in revenge for him spending too much money "playing the market and then you lost it all!"
Take a look at sensory balance. Most of us have a preferred sense, and if we're not careful we write to that sense. Visual writers give us vivid descriptions of sunsets, sunrises, mountains, waves on the shore, clothing...all with no tactile, auditory, or olfactory input. Their books are like watching TV with subtitles for sounds. Auditory writers are great with crash, bang, swish, plop...but may not include visual specifics...like navigating through fog. And most of us have to be reminded that we feel with our skin, smell with our noses and mouths, taste with our mouths and noses. We experience emotions viscerally--inside our bodies--and reveal them to others with visible changes, sounds we make, even smells. If you're a one-sense writer, you can enrich the experience for readers by adding sensory detail in other senses...not all one other sense, and not all in one place or everything everywhere.
Also look at conversations. Human communication is not just words--it includes behaviors that range from posture (and postural changes) to facial expressions, gestures, pitch/tone/prosody/rate of speech, and context (where and when the conversation occurs.) Does every conversation have its behavioral component? Even on the telephone, the POV speaker will lean into the phone, lean away, sit up or slump in the chair, walk faster or slower if on a cellphone. Adding a little more to the setting of a conversation, a little more behavioral component, enriches at the same time that wordage grows.
Except for structural additions (when you've left part of the story skeleton out, or need a larger one) most additions can and should be "feathered" in here and there, where opportunities for enrichment occur or more muscle is needed on the bone. Giving "skimpy" team member Rob more motivational levels, for instance, would be done bit by bit, at the same rate you developed the others, revealing motivation as the character acts. As long as you think of it as enrichment--as long as you add substance that enhances the story--adding isn't padding.