Find Familiar1
As an action, if your familiar is within 100', you can “enter” your familiar’s senses per the spell description. Dropping your connection to your familiar is a free action, and you do not need to use your action each round to maintian use of your familiar's senses. You can continue to see, hear and utilize any of your familiar's special senses (i.e. blindsense) up to a distance of 1000', but you cannot communicate telepathically with your familiar beyond 100' per the spell description.2
In this way, a spellcaster could utilize his or her familiar's sight in order to see creatures or locations they could not otherwise see, but this feature does not extend the range of a spell— a spell with a range of "60 feet" is based on distance from the caster, not the familiar. The obvious exception to this is touch spells, as detailed in the spell description.
You can choose to roll initiative for your familiar, or place its turn immediately after your own in the turn order.
Footnotes
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Ruling based on Matt Mercer's adapted familiar rules for Frumpkin and Grand Duchess Anastasia in Critical Role Season 2. ↩