Do-it-yourself enthusiasts are tackling an increasing variety of home projects these days, but one area that is still foreign territory to many home handymen and handywomen is the reupholstering of chairs.

One reason is that the techniques seem beyond the scope of the uninitiated, but an equally important reason is probably that the supplies and materials needed are not readily available in local stores.

Both these problems have been solved by the recently introduced Chair-reNew kit. It contains everything needed to reupholster almost any style living room or occasional chair - except for the fabric that you will want to use for the outer covering (additional cushioning fil may also be required for some chairs). The kit includes cotton batting, webbing, welting cord, tacks, twine, burlap, muslin, cambric, spring, edging, spring clips and tacking strips. Even the specialized professional-quality tools needed are included: magnetic tack hammer, webbing stretcher, upholstery needles, etc. - plus a book with step-by-step instructions.

The kit is packed in a handy cardboard carrying case, and costs $34.95, plus $3 for postage and handling, from Sackner Products, Inc., 901 Ottawa NW, Grand Rapids, Mich. 49503. The company is a nationwide supplier of upholstery materials to manufacturers of upholstered furniture. Portable Drilling Guide

Designed so it can be used with almost standard brand 1/4-inch or 3/8-inch portable electric drill, the new-Portalign Precision Drill Guide has the accuracy of a drill press. Made by the Portalign Tool Corp., 4903 Pacific Hwy., San Diego, Calif. 92110, and priced at $20, the guide is used by removing the chuck from the drill, then re-attaching the chuck to the spindle end. The upper end of the spindle then fits into the chuck hole.

When the base plate is flat against, the surface, a perfect 90-degree hole is bored. When the guide rods are extended through the base, precision angle drilling is possible, and a depth stop provides precise control of depth repeatedly. In addition, U-blocks on the base plate make it easy to hold round stock and to center holes on it. The base plate can also be fastened to the under side of a bench top to hold it stationary (upside down) for sanding and shaping while the work is manipulated with the hands. Air Conditioner Timer

People who have room air conditioners often leave them running for hours when they are not at home on very hot days because they do not want to come home to hot, uncomfortable house or apartment. Now there is a special air conditioner timer that can be set to automatically turn any room air conditioner on and off once, twice, or more often each day. Called the Time-All Air Conditioner Timer, it is made by Intermatic, Inc., Spring Grove, Ill., 60031. It can be connected to a wall outlet by plugging its cord in, after which the air conditioner is plugged into the timer's plug. Available in 125-bolt model (EB41, $17.95), and 220-volt model (EB42, $18.95), it comes with two sets of trippers. As an example, the timer can be set to turn the unit off a couple of hours before you wake up in the morning, they turn it back on a couple of hours before you return in the evening. Considerable savings in the amount of electricity consumed will result, and you will always wake up to and come home to a comfortable temperature.