Indra frequently sends down Apsaras for men who try to do lots of Tapas to distract them.
Suppose one wants to intentionally get an Apsara. How much Tapas should one do for Indra to send down an Apsara for him?
There is something called Apsara Sadhana in Hinduism particularly in the DAmara Tantras. By which one can gain control over the Apsaras. Apsaras will come fulfill the aspirant's wishes and also stay with him as his wife (BhAryA).
Therefore, Indra's favor or intervention will not be needed.
One such Tantric scripture is the Bhuta DAmara Tantram.
The 10th Chapter of this text is dedicated completely to Apsara Sadhana.
The aspirant has to practice the Mantra of the desired Apsara and achieve Siddhi (mastery) in it. The Prayogas and details related to this practice are found in the same text.
The Chapter ends with the line:
Iti Bhuta DAmare Apsarah SAdhanam NAma Dashamah Patalah | Thus ends the 10th Chapter named Apsarah Sadhana in the Bhuta DAmara Tantram
Mantras, Prayogas, MudrAs etc are given in this chapter for seven Apsaras including some well-known ones like Urvashi and RambhA.
PrAleyam shri shashi devyA anAdi shri tilottamA |
SAnAdim shrim manum smritya yuktam kAnchanmAlaya |
Visham shri varma samyuktamAbhAshya kulahArini ||
TAram varna samAyuktAm ratnamAleti panchami |
TAm sa iti rambhAkhyAm visham shrim urvashi parA | .... (10.3)
The above verses are containing, in coded forms, the Mantras for the 7 Apasaras viz: Tilottama, KAnchanmAlA, KulahArini, RatnamAlA, RambhA, Urvashi and RamAbhushani.
Atha vakshyeha apsaro vasyokArakam manumuttam |
Visham chaladvaram proktvA amukim vashamAnaya |
Now, the mantra for gaining control over the Apsaras are being spoken. By chanting this mantra the Apsaras remain controlled (Vashibhuta).
If the aspirant is successful in the SAdhanA, then in the night time the Apsara comes to the aspirant's place and becomes his wife and presents him with gemstones, gold ornaments, heavenly food etc etc..
During the days, in which the aspirant is getting her favor, he should entirely forsake cohabiting with any other women. Otherwise, he has to face the wrath of the Apsara.